Chapter C: O. H. Cl. Br. I. S (1)

11 12.2 5.5 22.8 27.8 37.5 22.6.

These values hold fairly well when compared with the experimental
values determined from other compounds, and also with the molecular
volumes of the elements themselves. Thus the actually observed
densities of liquid chlorine and bromine at the boiling-points are
1.56 and 2.96, leading to atomic volumes 22.7 and 26.9, which closely
correspond to Kopp's values deduced from organic compounds.

These values, however, require modification in certain cases, for
discrepancies occur which can be reconciled in some cases by assuming
that the atomic value of a polyvalent element varies according to the
distribution of its valencies. Thus a double bond of oxygen, as in the
carbonyl group CO, requires a larger volume than a single bond, as in
the hydroxyl group -OH, being about 12.2 in the first case and 7.8 in
the second. Similarly, an increase of volume is associated with doubly
and trebly linked carbon atoms.

Recent researches have shown that the law originally proposed by
Kopp--"That the specific volume of a liquid compound (molecular
volume) at its boiling-point is equal to the sum of the specific
volumes of its constituents (atomic volumes), and that every element
has a definite atomic value in its compounds"--is by no means exact,
for isomers have different specific volumes, and the volume for an
increment of CH2 in different homologous series is by no means
constant; for example, the difference among the esters of the fatty
acids is about 57, whereas for the aliphatic aldehydes it is 49. We
may therefore conclude that the molecular volume depends more upon the
internal structure of the molecule than its empirical content. W.
Ostwald (_Lehr. der allg. Chem._), after an exhaustive review of the
material at hand, concluded that simple additive relations did exist
but with considerable deviations, which he ascribed to differences in
structure. In this connexion we may notice W. Stadel's determinations:

CH3CCl3 108 CHClBr.CH3 96.5
CH2Cl.CHCl2 102.8 CH2Br.CH2Cl 88

These differences do not disappear at the critical point, and hence
the critical volumes are not strictly additive.

Theoretical considerations as to how far Kopp was justified in
choosing the boiling-points under atmospheric pressure as being
comparable states for different substances now claim our attention.
Van der Waal's equation (p+a/v^2)(v-b) = RT contains two constants a
and b determined by each particular substance. If we express the
pressure, volume and temperature as fractions of the critical
constants, then, calling these fractions the "reduced" pressure,
volume and temperature, and denoting them by [pi], [phi] and [theta]
respectively, the characteristic equation becomes
([pi]+3/[phi]^2)(3[phi]-1) = 8[theta]; which has the same form for all
substances. Obviously, therefore, liquids are comparable when the
pressures, volumes and temperatures are equal fractions of the
critical constants. In view of the extremely slight compressibility of
liquids, atmospheric pressure may be regarded as a coincident
condition; also C.M. Guldberg pointed out that for the most diverse
substances the absolute boiling-point is about two-thirds of the
critical temperature. Hence within narrow limits Kopp's determinations
were carried out under coincident conditions, and therefore any
regularities presented by the critical volumes should be revealed in
the specific volumes at the boiling-point.

Volume relations of solids.

The connexion between the density and chemical composition of solids
has not been investigated with the same completeness as in the case of
gases and liquids. The relation between the atomic volumes and the
atomic weights of the solid elements exhibits the periodicity which
generally characterizes the elements. The molecular volume is additive
in certain cases, in particular of analogous compounds of simple
constitution. For instance, constant differences are found between the
chlorides, bromides and iodides of sodium and potassium:--

+------------+------+-------------+------+----------------+
| I. |Diff. | II. | Diff.| Diff. I. & II. |
+------------+------+-------------+------+----------------+
| KCl = 37.4 | 6.9 | NaCl = 27.1 | 6.7 | 10.3 |
| KBr = 44.3 | 9.7 | NaBr = 33.8 | 9.7 | 10.5 |
| KI = 54.0 | | NaI = 43.5 | | 10.5 |
+------------+------+-------------+------+----------------+

According to H. Schroeder the silver salts of the fatty acids exhibit
additive relations; an increase in the molecule of CH2 causes an
increase in the molecular volume of about 15.3.

_Thermal Relations._

_Specific Heat and Composition._---The nature and experimental determination of specific heats are discussed in the article CALORIMETRY; here will be discussed the relations existing between the heat capacities of elements and compounds.

Specific heat of gases.

In the article THERMODYNAMICS it is shown that the amount of heat required to raise a given weight of a gas through a certain range of temperature is different according as the gas is maintained at constant pressure, the volume increasing, or at constant volume, the pressure increasing. A gas, therefore, has two specific heats, generally denoted by C_p and C_v, when the quantity of gas taken as a unit is one gramme molecular weight, the range of temperature being 1 deg. C. It may be shown that C_p - C_v = R, where R is the gas-constant, i.e. R in the equation PV = RT. From the ratio C_p/C_v conclusions may be drawn as to the molecular condition of the gas. By considerations based on the kinetic theory of gases (see MOLECULE) it may be shown that when no energy is utilized in separating the atoms of a molecule, this ratio is 5/3 = 1.67. If, however, an amount of energy a is taken up in separating atoms, the ratio is expressible as C_p/C_v = (5+a)/(3+a), which is obviously smaller than 5/3, and decreases with increasing values of a. These relations may be readily tested, for the ratio C_p/C_v is capable of easy experimental determination. It is found that mercury vapour, helium, argon and its associates (neon, krypton, &c.) have the value 1.67; hence we conclude that these gases exist as monatomic molecules. Oxygen, nitrogen, hydrogen and carbon monoxide have the value 1.4; these gases have diatomic molecules, a fact capable of demonstration by other means. Hence it may be inferred that this value is typical for diatomic molecules. Similarly, greater atomic complexity is reflected in a further decrease in the ratio C_p/C_v. The following table gives a comparative view of the specific heats and the ratio for molecules of variable atomic content.

The abnormal specific heats of the halogen elements may be due to a
loosening of the atoms, a preliminary to the dissociation into
monatomic molecules which occurs at high temperatures. In the more
complex gases the specific heat varies considerably with temperature;
only in the case of monatomic gases does it remain constant. Le
Chatelier (_Zeit. f. phys. Chem._ i. 456) has given the formula C_p
= 6.5 + aT, where a is a constant depending on the complexity of the
molecule, as an expression for the molecular heat at constant pressure
at any temperature T (reckoned on the absolute scale). For a further
discussion of the ratio of the specific heats see MOLECULE.

+-----------+-------------------------------+-------+-------+---------+
| Molecular | Examples. | C_p | C_v | C_p/C_v |
| Content. | | | | |
+-----------+-------------------------------+-------+-------+---------+
| Monatomic | Hg, Zn, Cd, He, Ar, &c. | 5 | 3 | 1.66 |
+-----------+-------------------------------+-------+-------+---------+
| | H2, 02, N2 (0 deg.-200 deg.) | 6.83 | 4.83 | 1.41 |
| Diatomic | Br2, I2 (0 deg.-200 deg.) | 8.6 | 6.6 | 1.30 |
| | Cl2,HCl, HBr, HI, NO, CO | | | 1.41 |
+-----------+-------------------------------+-------+-------+---------+
| Triatomic | H2O, H2S, N2O, CO2 | 9.2 | 7.2 | 1.28 |
+-----------+-------------------------------+-------+-------+---------+
| Tetratomic| As4, P4 | 13.4 | 11.4 | 1.175 |
| | NH3, C2H2 | 11.6 | 9.6 | 1.21 |
+-----------+-------------------------------+-------+-------+---------+
| Pentatomic| CHCl3 | 14 | 12 | 1.17 |
+-----------+-------------------------------+-------+-------+---------+
| Hexatomic | C2H4, C2H3Br | 16.4 | 14.4 | 1.14 |
+-----------+-------------------------------+-------+-------+---------+

_Specific Heats of Solids._--The development of the atomic theory and the subsequent determination of atomic weights in the opening decades of the 19th century inspired A.T. Petit and P.L. Dulong to investigate relations (if any) existing between specific heats and the atomic weight. Their observations on the solid elements led to a remarkable generalization, now known as Dulong and Petit's law. This states that "the atomic heat (the product of the atomic weight and specific heat) of all elements is a constant quantity." The value of this constant when H = 1 is about 6.4; Dulong and Petit, using O = 1, gave the value .38, the specific heat of water being unity in both cases. This law--purely empirical in origin--was strengthened by Berzelius, who redetermined many specific heats, and applied the law to determine the true atomic weight from the equivalent weight. At the same time he perceived that specific heats varied with temperature and also with allotropes, e.g. graphite and diamond. The results of Berzelius were greatly extended by Hermann Kopp, who recognized that carbon, boron and silicon were exceptions to the law. He regarded these anomalies as solely due to the chemical nature of the elements, and ignored or regarded as insignificant such factors as the state of aggregation and change of specific heat with temperature.

The specific heats of carbon, boron and silicon subsequently formed
the subject of elaborate investigations by H.F. Weber, who showed that
with rise of temperature the specific (and atomic) heat increases,
finally attaining a fairly constant value; diamond, graphite and the
various amorphous forms of carbon having the value about 5.6 at 1000
deg., and silicon 5.68 at 232 deg.; while he concluded that boron
attained a constant value of 5.5. Niison and Pettersson's observations
on beryllium and germanium have shown that the atomic heats of these
metals increase with rise of temperature, finally becoming constant
with a value 5.6. W.A. Tilden (_Phil. Trans._, 1900, p. 233)
investigated nickel and cobalt over a wide range of temperature (from
-182.5 deg. to 100 deg.); his results are:--

+--------------------------------+---------+---------+
| | Cobalt. | Nickel. |
+--------------------------------+---------+---------+
| From -182.5 deg. to -78.4 deg. | 4.1687 | 4.1874 |
| -78.4 deg. to 15 deg. | 5.4978 | 5.6784 |
| 15 deg. to 100 deg. | 6.0324 | 6.3143 |
+--------------------------------+---------+---------+

It is evident that the atomic heats of these intimately associated
elements approach nearer and nearer as we descend in temperature,
approximating to the value 4. Other metals were tested in order to
determine if their atomic heats approximated to this value at low
temperatures, but with negative results.

It is apparent that the law of Dulong and Petit is not rigorously
true, and that deviations are observed which invalidate the law as
originally framed. Since the atomic heat of the same element varies
with its state of aggregation, it must be concluded that some factor
taking this into account must be introduced; moreover, the variation
of specific heat with temperature introduces another factor.

We now proceed to discuss molecular heats of compounds, that is, the product of the molecular weight into the specific heat. The earliest generalization in this direction is associated with F.E. Neumann, who, in 1831, deduced from observations on many carbonates (calcium, magnesium, ferrous, zinc, barium and lead) that stoichiometric quantities (equimolecular weights) of compounds possess the same heat capacity. This is spoken of as "Neumann's law." Regnault confirmed Neumann's observations, and showed that the molecular heat depended on the number of atoms present, equiatomic compounds having the same molecular heat. Kopp systematized the earlier observations, and, having made many others, he was able to show that the molecular heat was an additive property, i.e. each element retains the same heat capacity when in combination as in the free state. This has received confirmation by the researches of W.A. Tilden (_Phil. Trans._, 1904, 203 A, p.139) for those elements whose atomic heats vary considerably with temperature.

The specific heat of a compound may, in general, be calculated from
the specific heats of its constituent elements. Conversely, if the
specific heats of a compound and its constituent elements, except one,
be known, then the unknown atomic heat is readily deducible.
Similarly, by taking the difference of the molecular heats of
compounds differing by one constituent, the molecular (or atomic) heat
of this constituent is directly obtained. By this method it is shown
that water, when present as "water of crystallization," behaves as if
it were ice.

_Deductions from Dulong and Petit's Law._--Denoting the atomic weight by W and the specific heat by s, Dulong and Petit's law states that 6.4 = Ws. Thus if s be known, an approximate value of W is determinate. In the determination of the atomic weight of an element two factors must be considered: (1) its equivalent weight, i.e. the amount which is equivalent to one part of hydrogen; and (2) a factor which denotes the number of atoms of hydrogen which combines with or is equivalent to one atom of the particular element. This factor is termed the valency. The equivalent weight is capable of fairly ready determination, but the settlement of the second factor is somewhat more complex, and in this direction the law of atomic heats is of service. To take an example: 38 parts of indium combine with 35.4 parts of chlorine; hence, if the formula of the chloride be InCl, InCl2 or InCl3, indium has the atomic weights 38, 76 or 114. The specific heat of indium is 0.057; and the atomic heats corresponding to the atomic weights 38, 76 and 114 are 3.2, 4.3, 6.5. Dulong and Petit's law thus points to the value 114, which is also supported by the position occupied by this element in the periodic classification. C. Winkler decided the atomic weight of germanium by similar reasoning.

_Boiling-Point and Composition._--From the relation between the critical constants P_kV_k/T_k = (1/3.7)R or T_k/P_k = 3.7V_k/R, and since V_k is proportional to the volume at absolute zero, the ratio T_k/P_k should exhibit additive relations. This ratio, termed by Guye the critical coefficient, has the following approximate values:--

Double Triple
C. H. Cl. -O-. =O. N. N=. P. linkage. linkage.
1.35 0.57 2.66 0.87 1.27 1.6 1.86 3.01 0.88 1.03

Since at the boiling-point under atmospheric pressure liquids are in corresponding states, the additive nature of the critical coefficient should also be presented by boiling-points. It may be shown theoretically that the absolute boiling-point is proportional to the molecular volume, and, since this property is additive, the boiling-point should also be additive.

These relations have been more thoroughly tested in the case of
organic compounds, and the results obtained agree in some measure with
the deductions from molecular volumes. In general, isomers boil at
about the same temperature, as is shown by the isomeric esters
C9H18O2:--

Methyl octoate 192.9 deg. Amyl butyrate 184.8 deg.
Ethyl heptoate 187.1 deg. Heptyl acetate 191.3 deg.
Propyl hexoate 185.5 deg. Octyl formate 198.1 deg.
Butyl pentoate 185.8

Equal increments in the molecule are associated with an equal rise in
the boiling-point, but this increment varies in different homologous
series. Thus in the normal fatty alcohols, acids, esters, nitriles and
ketones, the increment per CH2 is 19 deg.-21 deg.; in the aldehydes it
is 26 deg.-27 deg.. In the aromatic compounds there is no regularity
between the increments due to the introduction of methyl groups into
the benzene nucleus or side chains; the normal value of 20 deg.-21
deg. is exhibited, however, by pyridine and its derivatives. The
substitution of a hydrogen atom by the hydroxyl group generally
occasions a rise in boiling-point at about 100 deg.. The same increase
accompanies the introduction of the amino group into aromatic nuclei.

Constitutive influences.

While certain additive relations hold between some homologous series,
yet differences occur which must be referred to the constitution of
the molecule. As a general rule, compounds formed with a great
evolution of heat have high boiling-points, and vice versa. The
introduction of negative groups into a molecule alters the
boiling-point according to the number of negative groups already
present. This is shown in the case of the chloracetic acids:

Diff.
CH3CO2H = 118 deg.
67 deg.
ClCH2.CO2H = 185 deg.
10 deg.
Cl2CH.CO2H = 195 deg.
3 deg.
Cl3C.CO2H = 195 deg.-200 deg.

According to van 't Hoff the substitution of chlorine atoms into a
methyl group occasions the following increments:--

Cl in CH3 66 deg.
Cl " CH2Cl 39 deg.
Cl " CHCl2 13 deg.

The introduction of chlorine, however, may involve a fall in the
boiling-point, as is recorded by Henry in the case of the chlorinated
acetonitriles:--

NC.CH3. NC.CH2Cl. NC.CHC12. NC.CC13.
81 deg. 123 deg. 112 deg. 83 deg.
42 deg. -11 deg. -29 deg.

The replacement of one negative group by another is accompanied by a
change in the boiling-point, which is independent of the compound in
which the substitution is effected, and solely conditioned by the
nature of the replaced and replacing groups. Thus bromine and iodine
replace chlorine with increments of about 22 deg. and 50 deg.
respectively.

A factor of considerable importance in determining boiling-points of
isomers is the symmetry of the molecule. Referring to the esters
C9H18O2 previously mentioned, it is seen that the highest
boiling-points belong to methyl octoate and octyl formate, the least
symmetrical, while the minimum belongs to amyl butyrate, the most
symmetrical. The isomeric pentanes also exhibit a similar relation
CH3(CH2)4CH3 = 38 deg., (CH3)2CHC2H5 = 30 deg., (CH3)4C = 9.5 deg..
For a similar reason secondary alcohols boil at a lower temperature
than the corresponding primary, the difference being about 19 deg..
A.E. Earp (_Phil. Mag._, 1893 [5], 35, p. 458) has shown that, while
an increase in molecular weight is generally associated with a rise in
the boiling-point, yet the symmetry of the resulting molecule may
exert such a lowering effect that the final result is a diminution in
the boiling-point. The series H2S = -61 deg., CH3SH = 21 deg., (CH3)2S
= 41 deg. is an example; in the first case, the molecular weight is
increased and the symmetry diminished, the increase of boiling-point
being 82 deg.; in the second case the molecular weight is again
increased but the molecule assumes a more symmetrical configuration,
hence the comparatively slight increase of 20 deg.. A similar
depression is presented by methyl alcohol (67 deg.) and methyl ether
(-23 deg.).

Among the aromatic di-substitution derivatives the _ortho_ compounds
have the highest boiling-point, and the _meta_ boil at a higher, or
about the same temperature as the _para_ compounds. Of the
tri-derivatives the symmetrical compounds boil at the lowest
temperature, the asymmetric next, and the vicinal at the highest.

An ethylenic or double carbon union in the aliphatic hydrocarbons has,
apparently, the same effect on the boiling-point as two hydrogen
atoms, since the compounds C_{n}H_{2n+2} and C_{n}H_{2n} boil at about
the same temperature. An acetylenic or triple linkage is associated
with a rise in the boiling-point; for example, propargyl compounds
boil about 19.5 deg. higher than the corresponding propyl compound.

Certain regularities attend the corresponding property of the
melting-point. A rule applicable to organic compounds, due to Adolf v.
Baeyer and supported by F.S. Kipping (_Jour. Chem. Soc._, 1893, 63,
p.465) states, that the melting-point of any odd member of a
homologous series is lower than the melting-point of the even member
containing one carbon atom less. This is true of the fatty acid
series, and the corresponding ketones and alcohols, and also of the
succinic acid series. Other regularities exist, but generally with
many exceptions. It is to be noted that although the correlation of
melting-point with constitution has not been developed to such an
extent as the chemical significance of other physical properties, the
melting-point is the most valuable test of the purity of a substance,
a circumstance due in considerable measure to the fact that impurities
always tend to lower the melting-point.

_Heat of Combustion and Constitution._--In the article THERMOCHEMISTRY a general account of heats of formation of chemical compounds is given, and it is there shown that this constant measures the stability of the compound. In organic chemistry it is more customary to deal with the "heat of combustion," i.e. the heat evolved when an organic compound is completely burned in oxygen; the heat of formation is deduced from the fact that it is equal to the heats of formation of the products of combustion less the observed heat of combustion. The researches of Julius Thomsen and others have shown that in many cases definite conclusions regarding constitution can be drawn from quantitative measurements of the heats of combustion; and in this article a summary of the chief results will be given.

The identity of the four valencies of the carbon atom follows from the
fact that the heats of combustion of methane, ethane, propane,
trimethyl methane, and tetramethyl methane, have a constant difference
in the order given, viz. 158.6 calories; this means that the
replacement of a hydrogen atom by a methyl group is attended by a
constant increase in the heat of combustion. The same difference
attends the introduction of the methyl group into many classes of
compounds, for example, the paraffins, olefines, acetylenes, aromatic
hydrocarbons, alcohols, aldehydes, ketones and esters, while a
slightly lower value (157.1) is found in the case of the halogen
compounds, nitriles, amines, acids, ethers, sulphides and nitro
compounds. It therefore appears that the difference between the heats
of combustion of two adjacent members of a series of homologous
compounds is practically a constant, and that this constant has two
average values, viz. 158.6 and 157.1.

An important connexion between heats of combustion and constitution is
found in the investigation of the effect of single, double and triple
carbon linkages on the thermochemical constants. If twelve grammes of
amorphous carbon be burnt to carbon dioxide under constant volume, the
heat evolved (96.96 cal.) does not measure the entire thermal effect,
but the difference between this and the heat required to break down
the carbon molecule into atoms. If the number of atoms in the carbon
molecule be denoted by n, and the heat required to split off each atom
from the molecule by d, then the total heat required to break down a
carbon molecule completely into atoms is nd. It follows that the true
heat of combustion of carbon, i.e. the heat of combustion of one
gramme-atom, is 96.96 + d. The value of d can be evaluated by
considering the combustion of amorphous carbon to carbon monoxide and
carbon dioxide. In the first case the thermal effect of 58.58 calories
actually observed must be increased by 2d to allow for the heat
absorbed in splitting off two gramme-atoms of carbon; in the second
case the thermal effect of 96.96 must be increased by d as above. Now
in both cases one gramme-molecule of oxygen is decomposed, and the two
oxygen atoms thus formed are combined with two carbon valencies. It
follows that the thermal effects stated above must be equal, i.e.
58.58 + 2d = 96.96 + d, and therefore d = 38.38. The absolute heat of
combustion of a carbon _atom_ is therefore 135.34 calories, and this
is independent of the form of the carbon burned.

Consider now the combustion of a hydrocarbon of the general formula
C_{n}H_{2m}. We assume that each carbon atom and each hydrogen atom
contributes equally to the thermal effect. If [alpha] be the heat
evolved by each carbon atom, and [beta] that by each hydrogen atom,
the thermal effect may be expressed as H = n[alpha] + 2m[beta] - A,
where A is the heat required to break the molecule into its
constituent atoms. If the hydrocarbon be saturated, i.e. only contain
single carbon linkages, then the number of such linkages is 2n - m,
and if the thermal effect of such a linkage be X, then the term A is
obviously equal to (2n - m)X. The value of H then becomes H = n[alpha]
+ 2m[beta] - (2n - m)X or n[xi] + m[nu], where [xi] and [nu] are
constants. Let double bonds be present, in number p, and let the
energy due to such a bond be Y. Then the number of single bonds is 2n
- m - 2p, and the heat of combustion becomes H1 = n[xi] + m[nu] +
p(2X - Y). If triple bonds, q in number, occur also, and the energy of
such a bond be Z, the equation for H becomes

H = n[xi] + m[nu] + p(2X - Y) + q(3X - Z).

This is the general equation for calculating the heat of combustion of
a hydrocarbon. It contains four independent constants; two of these
may be calculated from the heats of combustion of saturated
hydrocarbons, and the other two from the combustion of hydrocarbons
containing double and triple linkages. By experiment it is found that
the thermal effect of a double bond is much less than the effect of
two single bonds, while a triple bond has a much smaller effect than
three single bonds. J. Thomsen deduces the actual values of X, Y, Z to
be 14.71, 13.27 and _zero_; the last value he considers to be in
agreement with the labile equilibrium of acetylenic compounds. One of
the most important applications of these values is found in the case
of the constitution of benzene, where Thomsen decides in favour of the
Claus formula, involving nine single carbon linkages, and rejects the
Kekule formula, which has three single and three double bonds (see
section IV.).

The thermal effects of the common organic substituents have also been
investigated. The thermal effect of the "alcohol" group C.OH may be
determined by finding the heat of formation of the alcohol and
subtracting the thermal effects of the remaining linkages in the
molecule. The average value for primary alcohols is 44.67 cal., but
many large differences from this value obtain in certain cases. The
thermal effects increase as one passes from primary to tertiary
alcohols, the values deduced from propyl and isopropyl alcohols and
trimethyl carbinol being:--primary = 45.08, secondary = 50.39,
tertiary = 60.98. The thermal effect of the aldehyde group has the
average value 64.88 calories, i.e. considerably greater than the
alcohol group. The ketone group corresponds to a thermal effect of
53.52 calories. It is remarkable that the difference in the heats of
formation of ketones and the paraffin containing one carbon atom less
is 67.94 calories, which is the heat of formation of carbon monoxide
at constant volume. It follows therefore that two hydrocarbon radicals
are bound to the carbon monoxide residue with the same strength as
they combine to form a paraffin. The average value for the carboxyl
group is 119.75 calories, i.e. it is equal to the sum of the thermal
effects of the aldehyde and carbonyl groups.

The thermal effects of the halogens are: chlorine = l5.13 calories,
bromine = 7.68; iodine = -4.25 calories. It is remarkable that the
position of the halogen in the molecule has no effect on the heat of
formation; for example, chlorpropylene and allylchloride, and also
ethylene dichloride and ethylidene dichloride, have equal heats of
formation. The thermal effect of the ether group has an average value
of 34.31 calories. This value does not hold in the case of methylene
oxide if we assign to it the formula H2C.[O.CH2], but if the formula
H2C.O.CH2 (which assumes the presence of two free valencies) be
accepted, the calculated and observed heats of formation are in
agreement.

The combination of nitrogen with carbon may result in the formation of
nitriles, cyanides, or primary, secondary or tertiary amines. Thomsen
deduced that a single bond between a carbon and a nitrogen gramme-atom
corresponds to a thermal effect of 2.77 calories, a double bond to
5.44, and a treble bond to 8.31. From this he infers that cyanogen is
C:N.N:C and not N:C-C:N, that hydrocyanic acid is HC.N, and
acetonitrile CH3.C:N. In the case of the amines he decides in favour
of the formulae

H2C:NH3 H2C H2C
\\ \\
NH2 NH.CH3
/ /
H3C H3C
primary, secondary, tertiary.

These involve pentavalent nitrogen. These formulae, however, only
apply to aliphatic amines; the results obtained in the aromatic series
are in accordance with the usual formulae.

_Optical Relations_.

_Refraction and Composition._--Reference should be made to the article REFRACTION for the general discussion of the phenomenon known as the refraction of light. It is there shown that every substance, transparent to light, has a definite refractive index, which is the ratio of the velocity of light _in vacuo_ to its velocity in the medium to which the refractive index refers. The refractive index of any substance varies with (1) the wave-length of the light; (2) with temperature; and (3) with the state of aggregation. The first cause of variation may be at present ignored; its significance will become apparent when we consider dispersion (_vide infra_).The second and third causes, however, are of greater importance, since they are associated with the molecular condition of the substance; hence, it is obvious that it is only from some function of the refractive index which is independent of temperature variations and changes of state (i.e. it must remain constant for the same substance at any temperature and in any form) that quantitative relations between refractivity and chemical composition can be derived.

The pioneer work in this field, now frequently denominated "spectro-chemistry," was done by Sir Isaac Newton, who, from theoretical considerations based on his corpuscular theory of light, determined the function (n^2-1), where n is the refractive index, to be the expression for the refractive power; dividing this expression by the density (d), he obtained (n^2-1)/d, which he named the "absolute refractive power." To P.S. Laplace is due the theoretical proof that this function is independent of temperature and pressure, and apparent experimental confirmation was provided by Biot and Arago's, and by Dulong's observations on gases and vapours. The theoretical basis upon which this formula was devised (the corpuscular theory) was shattered early in the 19th century, and in its place there arose the modern wave theory which theoretically invalidates Newton's formula. The question of the dependence of refractive index on temperature was investigated in 1858 by J.H. Gladstone and the Rev. T.P. Dale; the more simple formula (n-1)/d, which remained constant for gases and vapours, but exhibited slight discrepancies when liquids were examined over a wide range of temperature, being adopted. The subject was next taken up by Hans Landolt, who, from an immense number of observations, supported in a general way the formula of Gladstone and Dale. He introduced the idea of comparing the refractivity of equimolecular quantities of different substances by multiplying the function (n-1)/d by the molecular weight (M) of the substance, and investigated the relations of chemical grouping to refractivity. Although establishing certain general relations between atomic and molecular refractions, the results were somewhat vitiated by the inadequacy of the empirical function which he employed, since it was by no means a constant which depended only on the actual composition of the substance and was independent of its physical condition. A more accurate expression (n^2-1)/(n^2+2)d was suggested in 1880 independently and almost simultaneously by L.V. Lorenz of Copenhagen and H.A. Lorentz of Leiden, from considerations based on the Clausius-Mossotti theory of dielectrics.

Assuming that the molecules are spherical, R.J.E. Clausius and O.F.
Mossotti found a relation between the dielectric constant and the
space actually occupied by the molecules, viz. K = (1 + 2a)/(1 - a),
or a = (K - 1)/(K + 2), where K is the dielectric constant and a the
fraction of the total volume actually occupied by matter. According to
the electromagnetic theory of light K = N^2, where N is the refractive
index for rays of infinite wave-length. Making this substitution, and
dividing by d, the density of the substance, we obtain a/d = (N^2 -
1)/(N^2 + 2 )d. Since a/d is the real specific volume of the molecule,
it is therefore a constant; hence (N^2 - 1)/(N^2 + 2)d is also a
constant and is independent of all changes of temperature, pressure,
and of the state of aggregation. To determine N recourse must be made
to Cauchy's formula of dispersion (q.v.), n = A + B/[lambda]^2 +
C/[lambda]^4 + ... from which, by extrapolation, [lambda] becoming
infinite, we obtain N = A. In the case of substances possessing
anomalous dispersion, the direct measurement of the refractive index
for Hertzian waves of very long wave-length may be employed.

It is found experimentally that the Lorenz and Lorentz function holds fairly well, and better than the Gladstone and Dale formula. This is shown by the following observations of Ruhlmann on water, the light used being the D line of the spectrum:--

+------+------------+-----------------------+
| t. | (n - 1)/d. | (n^2 - 1)/(n^2 + 2)d. |
+------+------------+-----------------------+
| 0 | 0.3338 | 0.2061 |
| 10 | 0.3338 | 0.2061 |
| 20 | 0.3336 | 0.2061 |
| 90 | 0.3321 | 0.2059 |
| 100 | 0.3323 | 0.2061 |
+------+------------+-----------------------+

Eykmann's observations also support the approximate constancy of the Lorenz-Lorentz formula over wide temperature differences, but in some cases the deviation exceeds the errors of observation. The values are for the H[alpha] line:--

+----------------------------+--------------+-----------------------+
| Substance. | Temp. | (n^2 - 1)/(n^2 + 2)d. |
+----------------------------+--------------+-----------------------+
| Isosafrol, C10H10O2 | / 17.6 deg. | 0.2925 |
| | \ 141.2 deg. | 0.2962 |
| Diphenyl ethylene, C14H12 | / 22 deg. | 0.3339 |
| | \ 143.4 deg. | 0.3382 |
| Quinoline, C9H7N | / 16.2 deg. | 0.3187 |
| | \ 141 deg. | 0.3225 |
+----------------------------+--------------+-----------------------+

The empirical formula (n^2 - 1)/(n^2 + 0.4)d apparently gives more constant values with change of temperature than the Lorenz-Lorentz form. The superiority of the Lorenz-Lorentz formula over the Gladstone and Dale formula for changes of state is shown by the following observations of Bruhl (_Zeit. f. phys. Chem._, 1891, 71, p. 4). The values are for the D line:--

+-------------------+---------+---------------------+---------------------+
| | | Gladstone and Dale. | Lorenz and Lorentz. |
| Substance. | Temp. +----------+----------+---------+-----------+
| | | Vapour. | Liquid. | Vapour. | Liquid. |
+-------------------+---------+----------+----------+---------+-----------+
| Water | 10 deg. | 0.3101 | 0.3338 | 0.2068 | 0.2061 |
| Carbon disulphide | 10 deg. | 0.4347 | 0.4977 | 0.2898 | 0.2805 |
| Chloroform | 10 deg. | 0.2694 | 0.3000 | 0.1796 | 0.1790 |
+-------------------+---------+----------+----------+---------+-----------+

[Sidenote: Additive relations.]

Landolt and Gladstone, and at a later date J.W. Bruhl, have
investigated the relations existing between the refractive power and
composition. To Landolt is due the proof that, in general, isomers,
i.e. compounds having the same composition, have equal molecular
refractions, and that equal differences in composition are associated
with equal differences in refractive power. This is shown in the
following table (the values are for H[alpha]):

+------------------------------+--------+-----------------+--------+-----------+
| Substance. | Mol. | Substance. |Mol. | Diff. for |
| |Refract.| |Refract.| CH2. |
+------------------------------+--------+-----------------+--------+-----------+
| Ethylene chloride \ C2H4Cl2 | 20.96 | Acetic acid | 12.93 | / 4.49 |
| Ethylidene chloride/ | 21.08 | Propionic acid | 17.42 | \ |
| Fumaric acid \ C4H4O4 | 70.89 | Butyric acid | 22.01 | / 4.59 |
| Maleic acid / | 70.29 | | | \ |
| o-Cresol \ | 32.52 | Acetaldehyde | 11.50 | / 4.43 |
| m-Cresol } C7H8O | 32.56 | Propionaldehyde | 15.93 | \ |
| p-Cresol / | 32.57 | Butylaldehyde | 20.52 | / 4.59 |
+------------------------------+--------+-----------------+--------+-----------+

Additive relations undoubtedly exist, but many discrepancies occur
which may be assigned, as in the case of molecular volumes, to
differences in constitution. Atomic refractions may be obtained either
directly, by investigating the various elements, or indirectly, by
considering differences in the molecular refractions of related
compounds. The first method needs no explanation. The second method
proceeds on the same lines as adopted for atomic volumes. By
subtracting the value for CH2, which may be derived from two
substances belonging to the same homologous series, from the molecular
refraction of methane, CH4, the value of hydrogen is obtained;
subtracting this from CH2, the value of carbon is determined.
Hydroxylic oxygen is obtained by subtracting the molecular refractions
of acetic acid and acetaldehyde. Similarly, by this method of
differences, the atomic refraction of any element may be determined.
It is found, however, that the same element has not always the same
atomic refraction, the difference being due to the nature of the
elements which saturate its valencies. Thus oxygen varies according as
whether it is linked to hydrogen (hydroxylic oxygen), to two atoms of
carbon (ether oxygen), or to one carbon atom (carbonyl oxygen);
similarly, carbon varies according as whether it is singly, doubly, or
trebly bound to carbon atoms.

A table of the atomic refractions and dispersions of the principal
elements is here given:--

+--------------------------+--------+-------+--------+---------------+
| | | | | Dispersion |
| Element. |H[alpha]| D. |H[gamma]| H[gamma]- |
| | | | | H[alpha] |
+--------------------------+--------+-------+--------+---------------+
| Hydrogen | 1.103 | 1.051 | 1.139 | 0.036 |
| Oxygen, hydroxyl | 1.506 | 1.521 | 1.525 | 0.019 |
| Oxygen, ether | 1.655 | 1.683 | 1.667 | 0.012 |
| Oxygen, carbonyl | 2.328 | 2.287 | 2.414 | 0.086 |
| Chlorine | 6.014 | 5.998 | 6.190 | 0.176 |
| Bromine | 8.863 | 8.927 | 9.211 | 0.348 |
| Iodine |13.808 |14.12 | 14.582 | 0.774 |
| Carbon (singly bound) | 2.365 | 2.501 | 2.404 | 0.039 |
| Double linkage of carbon | 1.836 | 1.707 | 1.859 | 0.23 |
| Triple | 2.22 | | 2.41 | 0.19 |
| Nitrogen, singly bound | | | | |
| and only to carbon | 2.76 | | 2.95 | 0.19 |
+--------------------------+--------+-------+--------+---------------+

_Dispersion and Composition._---In the preceding section we have seen
that substances possess a definite molecular (or atomic) refraction
for light of particular wave-length; the difference between the
refractions for any two rays is known as the molecular (or atomic)
dispersion. Since molecular refractions are independent of temperature
and of the state of aggregation, it follows that molecular dispersions
must be also independent of these conditions; and hence quantitative
measurements should give an indication as to the chemical composition
of substances. This subject has been principally investigated by
Bruhl; he found that molecular dispersions of liquids and gases were
independent of temperature, and fairly independent of the state of
aggregation, but that no simple connexion exists between atomic
refractions and dispersions (see preceding table). He also showed how
changes in constitution effected dispersions to a far greater extent
than they did refractions; thus, while the atomic dispersion of carbon
is 0.039, the dispersions due to a double and treble linkage is 0.23
and 0.19 respectively.

_Colour and Constitution._--In this article a summary of the theories which have been promoted in order to connect the colour of organic compounds with their constitution will be given, and the reader is referred to the article COLOUR for the physical explanation of this property, and to VISION for the physiological and psychological bearings. A clear distinction must be drawn between colour and the property of dyeing; all coloured substances are not dyes, and it is shown in the article DYEING that the property of entering into chemical or physical combination with fibres involves properties other than those essential to colour. At the same time, however, all dyestuffs are coloured substances.

A survey of coloured substances led O.N. Witt in 1876 to formulate his
"chromophore-auxochrome" theory. On this theory colour is regarded as
due to the presence of a "chromophore," and dyeing power to an
"auxochrome"; the latter by itself cannot produce colour or dyeing
power, but it is only active in the presence of a chromophore, when it
intensifies the colour and confers the property of dyeing. The
principal chromophores are the azo, -N=N-, azoxy, =N2O, nitro, -NO2,
nitroso, -NO, and carbonyl, =CO, groups. The azo-group is particularly
active, both the aliphatic and aromatic compounds being coloured. The
simplest aliphatic compounds, such as diazo-methane, diazo-ethane, and
azo-formic acid, are yellow; the diamide of the latter acid is
orange-red. Of the aromatic compounds azo-benzene is bright
orange-red, and [alpha]-azo-naphthalene forms red needles or small
steel-blue prisms. The azo-group, however, has little or no colouring
effect when present in a ring system, such as in cinnolene,
phthalazine and tolazone. The nitro group has a very important action
mainly on account of the readiness with which it can be introduced
into the molecule, but its effect is much less than that of the azo
group. The colour produced is generally yellow, which, in accordance
with a general rule, is intensified with an increase in the number of
groups; compare, for example, mono-, di-and tri-nitrobenzene. The
nitroso group is less important. The colour produced is generally of a
greenish shade; for example, nitrosobenzene is green when fused or in
solution (when crystalline, it is colourless), and dinitrosoresorcin
has been employed as a dyestuff under the names "solid green" and
"chlorine." The carbonyl group by itself does not produce colour, but
when two adjacent groups occur in the molecule, as for example in the
a-diketones (such as di-acetyl and benzil), a yellow colour is
produced. It also acts as a chromogenic centre when double bonds or
ethylenic linkages are present, as in fluorene ketone or fluorenone. A
more complex chromophoric group is the triple ethylenic grouping

=C\
>C=,
=C/

the introduction of which was rendered necessary by the
discovery of certain coloured hydrocarbons. As a general rule,
hydrocarbons are colourless; the exceptions include the golden yellow
acenaphthylene, the red bidiphenylene-ethylene, and the derivatives of
fulvene

CH:CH \
>CH2,
CH:CH /

which have been discussed by J. Thiele (_Ber._, 1900, 33, p. 666).
This grouping is not always colour-producing, since diphenyl is
colourless.

The most important auxochromes are the hydroxyl (-OH) and amino (-NH2)
groups. According to the modern theory of auxochromic action, the
introduction of a group into the molecule is accompanied by some
strain, and the alteration in colour produced is connected with the
magnitude of the strain. The amino group is more powerful than the
hydroxyl, and the substituted amino group more powerful still; the
repeated substitution of hydroxyl groups sometimes causes an
intensification and sometimes a diminution of colour.

We may here notice an empirical rule formulated by Nietzski in
1879:--the simplest colouring substances are in the greenish-yellow
and yellow, and with increasing molecular weight the colour passes
into orange, red, violet, blue and green. This rule, however, is by no
means perfect. Examination of the absorption spectra of coloured
compounds shows that certain groupings displace the absorption bands
in one direction, and other groupings in the other. If the bands be
displaced towards the violet, involving a regression through the
colours mentioned above, the group is said to be "hypsochromic"; if
the reverse occurs the group is "bathochromic." It may be generally
inferred that an increase in molecular weight is accompanied by a
change in colour in the direction of the violet.

Auxochromic groups generally aid one another, i.e. the tint deepens as
the number of auxochromes increases. Also the relative position of the
auxochrome to the chromophore influences colour, the ortho-position
being generally the most powerful. Kauffmann (_Ber._, 1906, 39, p.
1959) attempted an evaluation of the effects of auxochromic groups by
means of the magnetic optical constants. The method is based on the
supposition that the magnetic rotation measures the strain produced in
the molecule by an auxochrome, and he arranges the groups in the
following order:--

.OCOCH3 .OCH3 .NHCOCH3 .NH2 .N(CH3)2 .N(C2H5)2
-0.260 1.459 1.949 3.821 8.587 8.816

The phenomena attending the salt formation of coloured and colouring
substances are important. The chromophoric groups are rarely strongly
acid or basic; on the other hand, the auxochromes are strongly acid or
basic and form salts very readily. Notable differences attend the
neutralization of the chromophoric and auxochromic groups. With basic
substances, the chromophoric combination with a colourless acid is
generally attended by a deepening in colour; auxochromic combination,
on the other hand, with a lessening. Examples of the first case are
found among the colourless acridines and quinoxalines which give
coloured salts; of the second case we may notice the colourless
hydrochloride and sulphate of the deep yellow o-aminobenzophenone.
With acid substances, the combination with "colourless" metals, i.e.
metals producing colourless salts with acids, is attended by colour
changes contrary to those given above, auxochromic combination being
accompanied by a deepening, and chromophoric by a lessening of the
tint.

Mention may be made of the phenomenon of halochromism, the name given
to the power of colourless or faintly-coloured substances of combining
with acids to form highly-coloured substances without the necessary
production of a chromophoric group. The researches of Adolf von Baeyer
and Villiger, Kehrmann, Kauffmann and others, show that this property
is possessed by very many and varied substances. In many cases it may
be connected with basic oxygen, and the salt formation is assumed to
involve the passage of divalent into tetravalent oxygen. It seems that
intermolecular change also occurs, but further research is necessary
before a sound theory can be stated.

_Quinone Theory of Colour._--A theory of colour in opposition to the
Witt theory was proposed by Henry Armstrong in 1888 and 1892. This
assumed that all coloured substances were derivatives of ortho- or
para-quinone (see QUINONES), and although at the time of its
promotion little practical proof was given, yet the theory found wide
acceptance on account of the researches of many other chemists. It
follows on this theory that all coloured substances contain either of
the groupings

____ ____
/----\ /----\
/ \ / \
==< >== or < >== ,
\______/ \\ /
\____/ \\___/
\\

the former being a para-quinonoid, the latter an ortho-quinonoid.
While very many coloured substances must obviously contain this
grouping, yet in many cases it is necessary to assume a simple
intermolecular change, while in others a more complex rearrangement of
bonds is necessary. Quinone, which is light yellow in colour, is the
simplest coloured substance on this theory. Hydrocarbons of similar
structure have been prepared by Thiele, for example, the orange-yellow
tetraphenyl-_para_-xylylene, which is obtained by boiling the bromide
C6H4[CBr(C6H5)2]2 with benzene and molecular silver. The quinonoid
structure of many coloured compounds has been proved experimentally,
as, for example, by Hewitt for the benzene-azo-phenols, and Hantzsch
for triaminotriphenyl methane and acridine derivatives; but, at the
same time, many substances cannot be so explained. A notable example
is provided by the phthaleins, which result by the condensation of
phthalic anhydride with phenols. In the free state these substances
are colourless, and were assumed to have the formula shown in 1.
Solution in dilute alkali was supposed to be accompanied by the
rupture of the lactone ring with the formation of the quinonoid salt
shown in 2.

/\ /\ O /\ /\
HO / \ / \ OH \\/ \ / \ OH
| | | | | | | |
| | | | | | | |
\ / \ / \ / \ /\\ / \ /
\/ C \/ \/ C \/
/ \ /
C6H< >O C6H4<
\ / \
CO COON3
(1) (2)

Baeyer (_Ber._, 1905, 38, p. 569) and Silberrad (_Journ. Chem. Soc._,
1906, 89, p. 1787) have disputed the correctness of this explanation,
and the latter has prepared melliteins and pyromelliteins, which are
highly-coloured compounds produced from mellitic and pyromellitic
acids, and which cannot be formulated as quinones. Baeyer has
suggested that the nine carbon atom system of xanthone may act as a
chromophore. An alternative view, due to Green, is that the oxygen
atom of the xanthone ring is tetravalent, a supposition which permits
the formulation of these substances as ortho-quinonoids.

The theories of colour have also been investigated by Hantzsch, who
first considered the nitro-phenols. On the chromophore-auxochrome
theory (the nitro group being the chromophore, and the hydroxyl the
auxochrome) it is necessary in order to explain the high colour of the
metallic salts and the colourless alkyl and aryl derivatives to assume
that the auxochromic action of the hydroxyl group is only brought
strongly into evidence by salt formation. Armstrong, on the other
hand, assumed an intermolecular change, thus:--

/\ /\
HO / \ OH / \ == O
| | ===> | |
| | | |
\ / NO2 \ / == NO2Na.
\/ \/

The proof of this was left for Hantzsch, who traced a connexion with
the nitrolic acids of V. Meyer, which are formed when nitrous acid
acts on primary aliphatic nitro compounds. Meyer formulated these
compounds as nitroximes or nitro-isnitroso derivatives, viz.
R.C(NO2)(NOH). Hantzsch explains the transformation of the colourless
acid into red salts, which on standing yield more stable, colourless
salts, by the following scheme:--

N
NOH // \ NO2Na
// R.C O //
R.C ===> \ / ===> R.C
\\ N \\
NO2 // \ NO
O ONa

Colourless, stable. Coloured, labile. Colourless, stable

He has also shown that the nitrophenols yield, in addition to the
colourless true nitrophenol ethers, an isomeric series of coloured
unstable quinonoid _aci_-ethers, which have practically the same
colour and yield the same absorption spectra as the coloured metallic
salts. He suggests that the term "quinone" theory be abandoned, and
replaced by the _Umlagerungs_ theory, since this term implies some
intermolecular rearrangement, and does not connote simply benzenoid
compounds as does "quinonoid." H. von Liebig (_Ann._, 1908, 360, p.
128), from a very complete discussion of triphenyl-methane
derivatives, concluded that the grouping

R R R
.. .. ..
A - A - A

was the only true organic chromophore, colour production, however,
requiring another condition, usually the closing of a ring.

The views as to the question of colour and constitution may be summarized as follows:--(1) The quinone theory (Armstrong, Gomberg, R. Meyer) regards all coloured substances as having a quinonoid structure. (2) The chromophore-auxochrome theory (Kauffmann) regards colour as due to the entry of an "auxochrome" into a "chromophoric" molecule. (3) If a colourless compound gives a coloured one on solution or by salt-formation, the production of colour may be explained as a particular form of ionization (Baeyer), or by a molecular rearrangement (Hantzsch). A dynamical theory due to E.C.C. Baly regards colour as due to "isorropesis" or an oscillation between the residual affinities of adjacent atoms composing the molecule.

_Fluorescence and Constitution._--The physical investigation of the phenomenon named fluorescence--the property of transforming incident light into light of different refrangibility--is treated in the article FLUORESCENCE. Researches in synthetical organic chemistry have shown that this property of fluorescence is common to an immense number of substances, and theories have been proposed whose purpose is to connect the property with constitution.

In 1897 Richard Meyer (_Zeit. physik. Chemie_, 24, p. 468) submitted
the view that fluorescence was due to the presence of certain
"fluorophore" groups; such groupings are the pyrone ring and its
congeners, the central rings in anthracene and acridine derivatives,
and the paradiazine ring in safranines. A novel theory, proposed by
J.T. Hewitt in 1900 (_Zeit. f. physik. Chemie_, 34, p. 1; _B.A.
Report_, 1903, p. 628, and later papers in the _Journ. Chem. Soc._),
regards the property as occasioned by internal vibrations within the
molecule conditioned by a symmetrical double tautomerism, light of one
wave-length being absorbed by one form, and emitted with a different
wave-length by the other. This oscillation may be represented in the
case of acridine and fluorescein as

CH CH CH
//\ //\ /\\ //\ /|\ /\\ //\ /\\ /\\
| | || | ==> | || | || | ==> | || | |
| | || | <== | || | || | <== | || | |,
\\/ \\/ \// \\/ \|/ \// \\/ \// \//
N N N

O O O O O
\\/ \ / \ / \OH HO/ \ / \ / \OH / \ / \ / \//
| | | | ==> | | | | ==> | | | |
| | | | <== | | | | <== | | | |
\ / \\/ \ / \ / \ / \ / \ / \// \ /
C C C
/ / \ /
C6H4.COOH C6H4 O C6H4.COOH
\ /
CO

This theory brings the property of fluorescence into relation
with that of colour; the forms which cause fluorescence being the
coloured modifications: ortho-quinonoid in the case of acridine,
para-quinonoid in the case of fluorescein. H. Kauffmann (_Ber._, 1900,
33, p. 1731; 1904, 35, p. 294; 1905, 38, p. 789; _Ann._, 1906, 344, p.
30) suggested that the property is due to the presence of at least two
groups. The first group, named the "luminophore," is such that when
excited by suitable aetherial vibrations emits radiant energy; the
other, named the "fluorogen," acts with the luminophore in some way or
other to cause the fluorescence. This theory explains the fluorescence
of anthranilic acid (o-aminobenzoic acid), by regarding the aniline
residue as the luminophore, and the carboxyl group as the fluorogen,
since, apparently, the introduction of the latter into the
non-fluorescent aniline molecule involves the production of a
fluorescent substance. Although the theories of Meyer and Hewitt do
not explain (in their present form) the behaviour of anthranilic acid,
yet Hewitt has shown that his theory goes far to explain the
fluorescence of substances in which a double symmetrical tautomerism
is possible. This tautomerism may be of a twofold nature:--(1) it may
involve the mere oscillation of linkages, as in acridine; or (2) it
may involve the oscillation of atoms, as in fluorescein. A theory of a
physical nature, based primarily upon Sir J.J. Thomson's theory of
corpuscles, has been proposed by J. de Kowalski (_Compt. rend._ 1907,
144, p. 266). We may notice that ethyl oxalosuccinonitrile is the
first case of a fluorescent aliphatic compound (see W. Wislicenus and
P. Berg, _Ber._, 1908, 41, p. 3757).