Chapter VIII: Carbon and the Hydrocarbons (2)

[16 bis] Hannay, in 1880, obtained diamonds by heating a mixture of
heavy liquid hydrocarbons (paraffin oils) with magnesium in a
thick iron tube. This investigation, however, was not repeated.

Moissan (Paris, 1893) produced diamonds artificially by means of the high temperature attained in the electrical furnace[17] by dissolving carbon in molten cast iron, and allowing the solution with an excess of carbon, to cool under the powerful pressure exerted by rapidly cooling the metal.[17 bis] K. Chroustchoff attained the same end by means of silver, which dissolves carbon to the extent of 6 p.c. at a high temperature. Rousseau, for the same purpose, heated carbide of calcium in the electric furnace. There is no doubt that all these investigators obtained the diamond as a transparent body, which burnt into CO_{2}, and possessed an exceptional hardness, but only in the form of a fine powder.

[17] The _electrical furnace_ is an invention of recent times, and
gives the possibility of obtaining a temperature of 3,500°, which
is not only not obtainable in ordinary furnaces, but even in the
oxyhydrogen flame, whose temperature does not exceed 2,000°. The
electrical furnace consists of two pieces of lime, laid one on
the other. A cavity is made in the lower piece for the reception
of the substance to be melted between two thick electrodes of
dense carbon. On passing a current of 70 volts and 450 ampères
a temperature of 3,000° is easily obtained. At a temperature of
2,500° (100 ampères and 40 volts) not only do all metals melt,
but even lime and magnesia (when placed in the space between
the carbon electrodes, _i.e._ in the voltaic arc) become soft
and crystallise on cooling. At 3,000° lime becomes very fluid,
metallic calcium partially separates out and a carbon compound,
which remains liquid for a long time. At this temperature oxide
of uranium is reduced to the suboxide and metal, zirconia and
rock crystal fuse and partially volatilise, as also does alumina;
platinum, gold, and even carbon distinctly volatilise; the
majority of the metals form carbides. At such a temperature also
cast iron and carbon give graphite, while according to Rousseau,
between 2,000° and 3,000° the diamond passes into graphite and
conversely graphite into the diamond, so that this is a kind of
reversible reaction.

[17 bis] Moissan first investigated the solution of carbon in molten
metals (and the formation of the carbides) such as magnesium,
aluminium, iron, manganese, chromium, uranium, silver, platinum,
and silicon. At the same time Friedel, owing to the discovery of
the diamond in meteoric iron, admitted that the formation of the
diamond is dependent upon the influence of iron and sulphur. With
this object, that is to obtain the diamond, Friedel caused sulphur
to react upon samples of cast iron rich in carbon, in a closed
vessel at a maximum temperature of 500°, and after dissolving the
sulphide of iron formed, he obtained a small quantity of a black
powder which scratched corundum, i.e. diamond. Moissan's
experiments (1893) were more successful, probably owing to his
having employed the electrical furnace. If iron be saturated with
carbon at a temperature between 1,100° and 3,000°, then at
1,100°-1,200° a mixture of amorphous carbon and graphite is
formed, while at 3,000° graphite alone is obtained in very
beautiful crystals. Thus under these conditions the diamond is not
formed, and it can only be obtained if the high temperature be
aided by powerful pressures. For this purpose Moissan took
advantage of the pressure produced in the passage of a mass of
molten cast iron from a liquid into a solid state. He first melted
150-200 grams of iron in the electrical furnace, and quickly
introduced a cylinder of carbon into the molten iron. He then
removed the crucible with the molten iron from the furnace and
plunged it into a reservoir containing water. After treating with
boiling hydrochloric acid, three varieties of carbon were
obtained: (1) a small amount of graphite (if the cooling be
rapid); (2) carbon of a chestnut colour in very fine twisted
threads, showing that it had been subjected to a very high
pressure (a similar variety was met with in various samples of the
Canon Diabolo), and lastly (3) an inconsiderable quantity of an
exceeding dense mass which was freed from the admixture of the
lighter modifications by treatment with _aqua regia_, sulphuric
and hydrofluoric acids, and from which Moissan, by means of liquid
bromoform (sp. gr. 2·900), succeeded in separating some small
pieces, having a greater density than bromoform, which scratched
the ruby and had the properties of the diamond. Some of these
pieces were black, others were transparent and refracted light
strongly. The dark grey tint of the former resembled that of the
black diamonds (carbonado). Their density was between 3 and 3·5.
The transparent specimens had a greasy appearance and seemed to
be, as it were, surrounded by an envelope of carbon. At 1,050°
they did not burn entirely in a current of air, so that the
imperfectly burnt particles, and a peculiar form of grains of a
light ochre colour, which retained their crystalline form, could
be examined under the microscope. Similar grains also remain after
the imperfect combustion of the ordinary diamond. Moissan obtained
the same results by rapidly cooling in a stream of coal gas a
piece of cast iron, saturated with carbon obtained from sugar and
first heated to 2,000°. In this instance he obtained small
crystals of diamonds. K. Chroustchoff showed that at its boiling
point silver dissolves 6 p.c. of carbon. This silver was rapidly
cooled, so that a crust formed on the surface and prevented the
metal expanding, and so produced a powerful pressure. A portion of
the carbon which separates out under these conditions exhibits the
properties of the diamond.

Judging from the fact that carbon forms a number of gaseous bodies (carbonic oxide, carbonic anhydride, methane, ethylene, acetylene, &c.) and volatile substances (for example, many hydrocarbons and their most simple derivatives), and considering that the atomic weight of carbon, C = 12, approaches that of nitrogen, N = 14, and that of oxygen, O = 16, and that the compounds CO (carbonic oxide) and N_{2}C_{2} (cyanogen) are gases, it may be argued that if carbon formed the molecule C_{2}, like N_{2} and O_{2}, it would be a gas. And as through polymerism or the combination of like molecules (as O_{2} passes into O_{3} or NO_{2} into N_{2}O_{4}) the temperatures of ebullition and fusion rise (which is particularly clearly proved with the hydrocarbons of the C_{n}H_{2n} series), it ought to be considered that _the molecules of charcoal, graphite, and the diamond are very complex_, seeing that they are insoluble, non-volatile, and infusible. The aptitude which the atoms of carbon show for combining together and forming complex molecules appears in all carbon compounds. Among the volatile compounds of carbon many are well known the molecules of which contain C_{5} ... C_{10} ... C_{20} ... C_{30}, &c., in general C_{n} where n may be very large, and in none of the other elements is this faculty of complexity so developed as in carbon.[18] Up to the present time there are no grounds for determining the degree of polymerism of the charcoal, graphite, or diamond molecules, and it can only be supposed that they contain C_{n} where n is a large quantity. Charcoal and those complex non-volatile organic substances which represent the gradual transitions to charcoal[19] and form the principal solid substances of organisms, contain a store or accumulation of internal power in the form of the energy binding the atoms into complex molecules. When charcoal or complex compounds of carbon burn, the energy of the carbon and oxygen is turned into heat, and this fact is taken advantage of at every turn for the generation of heat from fuel.[20]

[18] The existence of a molecule S_{6} is known (up to 600°), and it
must he beld that this accounts for the formation of hydrogen
persulphide, H_{2}S_{5}. Phosphorus appears in the molecule
P_{4} and gives P_{4}H_{2}. When expounding the data on specific
heat we shall have occasion to return to the question of the
complexity of the carbon molecule.

[19] The hydrocarbons poor in hydrogen and containing many atoms of
carbon, like chrysene and carbopetrocene, &c.,
C_{_n_}H_{2(_n_-_m_)}, are solids, and less fusible as _n_ and
_m_ increase. They present a marked approach to the properties
of the diamond. And in proportion to the diminution of the water
in the carbohydrates C_{_n_}H_{2_m_}O_{_m_}--for example in the
humic compounds (Note 5)--the transition of complex organic
substances to charcoal is very evident. That residue resembling
charcoal and graphite which is obtained by the separation (by
means of copper sulphate and sodium chloride) of iron from white
cast-iron containing carbon chemically combined with the iron,
also seems, especially after the researches of G. A. Zaboudsky,
to be a complex substance containing C_{12}H_{6}O_{3}. The
endeavours which have been directed towards determining the
measure of complexity of the molecules of charcoal, graphite, and
the diamond will probably at some period lead to the solution
of this problem and will most likely prove that the various
forms of charcoal, graphite, and the diamond contain molecules
of different and very considerable complexity. The constancy of
the grouping of benzene, C_{6}H_{6}, and the wide diffusion and
facility of formation of the carbohydrates containing C_{6} (for
example, cellulose, C_{6}H_{10}O_{5}, glucose, C_{6}H_{12}O_{6})
give reason for thinking that the group C_{6} is the first and
simplest of those possible to free carbon, and it may be hoped
that some time or other it may be possible to get carbon in this
form. Perhaps in the diamond there may be found such a relation
between the atoms as in the benzene group, and in charcoal such
as in carbohydrates.

[20] When charcoal burns, the complex molecule C_{_n_} is resolved into
the simple molecules _n_CO_{2}, and therefore part of the
heat--probably no small amount--is expended in the destruction of
the complex molecule C_{_n_}. Perhaps by burning the most complex
substances, which are the poorest as regards hydrogen, it may be
possible to form an idea of the work required to split up C_{_n_}
into separate atoms.

No other two elements are capable of combining together in such variety as carbon and hydrogen. The hydrocarbons of the C_{_n_}H_{2_m_} series in many cases differ widely from each other, although they have some properties in common. All hydrocarbons, whether gaseous, liquid or solid, are combustible substances sparingly soluble or insoluble in water. The liquefied gaseous hydrocarbons, as well as those which are liquid at ordinary temperatures, and those solid hydrocarbons which have been liquefied by fusion, have the appearance and property of oily liquors, more or less viscid, or fluid.[21] The solid hydrocarbons more or less resemble wax in their properties, although ordinary oils and wax generally contain oxygen in addition to carbon and hydrogen, but in relatively small proportion. There are also many hydrocarbons which have the appearance of tar--as, for instance, metacinnamene and gutta-percha. Those liquid hydrocarbons which boil at a high temperature are like oils, and those which have a low boiling point resemble ether, whilst the gaseous hydrocarbons in many of their properties are akin to hydrogen. All this tends to show that in hydrocarbons physically considered the properties of solid non-volatile charcoal are strongly modified and hidden, whilst those of the hydrogen predominate. All hydrocarbons are neutral substances (neither basic nor acid), but under certain conditions they enter into peculiar reactions. It has been seen in those hydrogen compounds which have been already considered (water, nitric acid, ammonia) that the hydrogen in almost all cases enters into reaction, being displaced by metals. The hydrogen of the hydrocarbons, it may be said, has no metallic character that is to say, it is not directly[22] displaced by metals, even by such as sodium and potassium. On the application of more or less heat all hydrocarbons decompose[23] forming charcoal and hydrogen. The majority of hydrocarbons do not combine with the oxygen of the air or oxidise at ordinary temperatures, but under the action of nitric acid and many other oxidising substances most of them undergo oxidation, in which either a portion of the hydrogen and carbon is separated, or the oxygen enters into combination, or else the elements of hydrogen peroxide enter into combination with the hydrocarbon.[24] When heated in air, hydrocarbons burn, and, according to the amount of carbon they contain, their combustion is attended more or less with a separation of soot--that is, finely divided charcoal--which imparts great brilliancy to the flame, and on this account many of them are used for the purposes of illumination--as, for instance, kerosene, coal gas, oil of turpentine. As hydrocarbons contain reducing elements (that is, those capable of combining with oxygen), they often act as reducing agents--as, for instance, when heated with oxide of copper, they burn, forming carbonic anhydride and water, and leave metallic copper. Gerhardt proved that all hydrocarbons contain an even number of hydrogen atoms. Therefore, the general formula for all hydrocarbons is C_{_n_}H_{2_m_} where _n_ and _m_ are whole numbers. This fact is known as _the law of even numbers_. Hence, the simplest possible hydrocarbons ought to be: CH_{2}, CH_{4}, CH_{6} ... C_{2}H_{2}, C_{2}H_{4}, C_{2}H_{6}, C_{2}H_{8} ... but they do not all exist, since the quantity of H which can combine with a certain amount of carbon is limited, as we shall learn directly.

[21] The viscosity, or degree of mobility, of liquids is determined
by their internal friction. It is estimated by passing the
liquids through narrow (capillary) tubes, the mobile liquids
passing through with greater facility and speed than the viscid
ones. The viscosity varies with the temperature and nature of the
liquids, and in the case of solutions changes with the amount of
the substance dissolved, but is not proportional to it. So that,
for example, with alcohol at 20° the viscosity will be 69, and
for a 50 p.c. solution 160, the viscosity of water being taken as
100. The volume of the liquid which passes through by experiment
(Poiseuille) and theory (Stokes) is proportional to the time, the
pressure, and the fourth power of the diameter of the (capillary)
tube, and inversely proportional to the length of the tube;
this renders it possible to form comparative estimates of the
coefficients of internal friction and viscosity.

As the complexity of the molecules of hydrocarbons and their
derivatives increases by the addition of carbon (or CH_{2}), so
does the degree of viscosity also rise. The extensive series
of investigations referring to this subject still await the
necessary generalisation. That connection which (already partly
observed) ought to exist between the viscosity and the other
physical and chemical properties, forces us to conclude that
the magnitude of internal friction plays an important part in
molecular mechanics. In investigating organic compounds and
solutions, similar researches ought to stand foremost. Many
observations have already been made, but not much has yet been
done with them; the bare facts and some mechanical data exist,
but their relation to molecular mechanics has not been cleared up
in the requisite degree. It has already been seen from existing
data that the viscosity at the temperature of the absolute
boiling point becomes as small as in gases.

[22] In a number of hydrocarbons and their derivatives such a
substitution of metals for the hydrogen may be attained by
indirect means. The property shown by acetylene, C_{2}H_{2},
and its analogues, of forming metallic derivatives is in this
respect particularly characteristic. Judging from the fact that
carbon is an acid element (that is, gives an acid anhydride with
oxygen), though comparatively slightly acid (for carbonic acid is
not at all a strong acid and compounds of chlorine and carbon,
even CCl_{4}, are not decomposed by water as is the case with
phosphorus chloride and even silicic chloride and boric chloride,
although they correspond with acids of but little energy),
one might expect to find in the hydrogen of hydrocarbons this
faculty for being substituted by metals. The metallic compounds
which correspond with hydrocarbons are known under the name of
organo-metallic compounds. Such, for instance, is zinc ethyl,
Zn(C_{2}H_{5})_{2}, which corresponds with ethyl hydride or
ethane, C_{2}H_{6}, in which two atoms of hydrogen have been
exchanged for one of zinc.

[23] Gaseous and volatile hydrocarbons decompose when passed through a
heated tube. When hydrocarbons are decomposed by heating, the
primary products are generally other more stable hydrocarbons,
among which are acetylene, C_{2}H_{2}, benzene, C_{6}H_{6},
naphthalene, C_{10}H_{8}, &c.

[24] Wagner (1888) showed that when unsaturated hydrocarbons are shaken
with a weak (1 p.c.) solution of potassium permanganate,
KMnO_{4}, at ordinary temperatures, they form glycols--for
example, C_{2}H_{4} yields C_{2}H_{6}O_{2}.

Some of the hydrocarbons are capable of combination, whilst others do not show that power. Those which contain less hydrogen belong to the former category, and those which, for a given quantity of carbon, contain the maximum amount of hydrogen, belong to the latter. The composition of those last mentioned is expressed by the general formula C_{_n_}H_{2_n_ + 2}. These so-called _saturated hydrocarbons_ are incapable of combination.[25] The hydrocarbons CH_{6}, C_{2}H_{8}, C_{3}H_{10}, &c.... do not exist. Those containing the maximum amount of hydrogen will be represented by CH_{4} (_n_ = 1, 2_n_ + 2 = 4), C_{2}H_{6} (_n_ = 2), C_{3}H_{8} (n = 3), C_{4}H_{10}, &c. This may be termed the _law of limits_. Placing this in juxtaposition with the law of even numbers, it is easy to perceive that the possible hydrocarbons can be ranged in series, the terms of which may be expressed by the general formulæ C_{_n_}H_{2_n_+2}, C_{_n_}H_{2_n_}, C_{_n_}H_{2_n_-2}, &c.... Those hydrocarbons which belong to any one of the series expressible by a general formula are said to be _homologous_0 with one another. Thus, the hydrocarbons CH_{4}, C_{2}H_{6}, C_{3}H_{8}, C_{4}H_{10}, &c.... are members of the limiting (saturated) homologous series C_{_n_}H_{2_n_+2}. That is, the difference between the members of the series is CH_{2}.[26] Not only the composition but also the properties of the members of a series tend to classification in one group. For instance, the members of the series C_{_n_}H_{2_n_+2} are not capable of forming additive compounds, whilst those of the series C_{_n_}H_{2_n_} are capable of combining with chlorine, sulphuric anhydride, &c.; and the members of the C_{_n_}H_{2_n_-6} group, belonging to the coal tar series, are easily nitrated (give nitro-compounds, Chapter VI.), and have other properties in common. The physical properties of the members of a given homologous series vary in some such manner as this; the boiling point generally rises and the internal friction increases as _n_ increases[27]--that is, with an increase in the relative amount of carbon and the atomic weight; the specific gravity also regularly changes as _n_ becomes greater.[28]

[25] My article on this subject appeared in the Journal of the
St. Petersburg Academy of Sciences in 1861. Up to that time,
although many additive combinations with hydrocarbons and their
derivatives were known, they had not been generalised, and were
even continually quoted as cases of substitution. Thus the
combination of ethylene, C_{2}H_{4}, with chlorine, Cl_{2}, was
often regarded as a formation of the products of the substitution
of C_{2}H_{5}Cl and HCl, which it was supposed were held together
as the water of crystallisation is in salts. Even earlier than
this (1857, _Journal of the Petroffsky Academy_) I considered
similar cases as true compounds. In general, according to the
law of limits, an unsaturated hydrocarbon, or its derivative, on
combining with _r_X_{2}, gives a substance which is saturated or
else approaching the limit. The investigations of Frankland with
many organo-metallic compounds clearly showed the limit in the
case of metallic compounds, which we shall constantly refer to
later on.

[26] The conception of homology has been applied by Gerhardt to all
organic compounds in his classical work, 'Traité de Chimie
Organique,' finished in 1855 (4 vols.), in which he divided
all organic compounds into _fatty_ and _aromatic_, which is in
principle still adhered to at the present time, although the
latter are more often called benzene derivatives, on account of
the fact that Kekulé, in his beautiful investigations on the
structure of aromatic compounds, showed the presence in them all
of the 'benzene nucleus,' C_{6}H_{6}.

[27] This is always true for hydrocarbons, but for derivatives of the
lower homologues the law is sometimes different; for instance,
in the series of saturated alcohols, C_{_n_}H_{2_n_+1}(OH),
when _n_ = 0, we obtain water, H(OH), which boils at 100°, and
whose specific gravity at 15° = 0·9992; when _n_ = 1, wood
spirit CH_{3}(OH), which boils at 66°, and at 15° has a specific
gravity = 0·7964; when _n_ = 2, ordinary alcohol, C_{2}H_{5}(OH),
boiling at 78°, specific gravity at 15° = 0·7936, and with
further increase of CH_{2} the specific gravity increases. For
the glycols C_{_n_}H_{2_n_}(OH)_{2} the phenomenon of a similar
kind is still more striking; at first the temperature of the
boiling point and the density increase, and then for higher (more
complex) members of the series diminish. The reason for this
phenomenon, it is evident, must be sought for in the influence
and properties of water, and that strong affinity which, acting
between hydrogen and oxygen, determines many of the exceptional
properties of water (Chapter I.).

[28] As, for example, in the saturated series of hydrocarbons
C_{_n_}H_{2_n_+2}, the lowest member (_n_ = 0) must be taken as
hydrogen H_{2}, a gas which (_t.c._ below -190°) is liquefied
with great difficulty, and when in a liquid state has doubtless
a very small density. Where _n_ = 1, 2, 3, the hydrocarbons
CH_{4}, C_{2}H_{6}, C_{3}H_{8} are gases, more and more readily
liquefiable. The temperature of the absolute boiling point for
CH_{4} =-100°, and for ethane C_{2}H_{6}, and in the higher
members it rises. The hydrocarbon C_{4}H_{10}, liquefies at about
0°. C_{5}H_{12} (there are several isomers) boils at from +9°
(Lvoff) to 37°, C_{6}H_{14} from 58° to 78°, &c. The specific
gravities in a liquid state at 15° are:--

C_{5}H_{12} C_{6}H_{14} C_{7}H_{16} C_{10}H_{22} C_{16}H_{34}
0·63 0·66 0·70 0·75 0·85

Many of the hydrocarbons met with in nature are the products of organisms, and do not belong to the mineral kingdom. A still greater number are produced artificially. These are formed by what is termed the combination of residues. For instance, if a mixture of the vapours of hydrogen sulphide and carbon bisulphide be passed through a tube in which copper is heated, this latter absorbs the sulphur from both the compounds, and the liberated carbon and hydrogen combine to form a hydrocarbon, methane. If carbon be combined with any metal and this compound MC_{_n_} be treated with an acid HX, then the haloid X will give a salt with the metal and the residual carbon and hydrogen will give a hydrocarbon. Thus cast iron which contains a compound of iron and carbon gives liquid hydrocarbons like naphtha under the action of acids. If a mixture of bromo-benzene, C_{6}H_{5}Br, and ethyl bromide, C_{2}H_{5}Br, be heated with metallic sodium, the sodium combines with the bromine of both compounds, forming sodium bromide, NaBr. From the first combination the group C_{6}H_{5} remains, and from the second C_{2}H_{5}. Having an odd number of hydrogen atoms, they, in virtue of the law of even numbers, cannot exist alone, and therefore combine together forming the compound C_{6}H_{5}.C_{2}H_{5} or C_{8}H_{10} (ethylbenzene). Hydrocarbons are also produced by the breaking up of more complex organic or hydrocarbon compounds, especially by heating--that is, by dry distillation. For instance, gum-benzoin contains an acid called benzoic acid, C_{7}H_{6}O_{2}, the vapours of which, when passed through a heated tube, split up into carbonic anhydride, CO_{2}, and benzene, C_{6}H_{6}. Carbon and hydrogen only unite directly in one ratio of combination--namely, to form acetylene, having the composition C_{2}H_{2}, which, as compared with other hydrocarbons, exhibits a very great stability at a somewhat high temperature.[29]

[29] If, at the ordinary temperature (assuming therefore that the
water formed will be in a liquid state) a gram molecule (26
grams) of acetylene, C_{2}H_{2}, be burnt, 310 thousand calories
will be emitted (Thomsen), and as 12 grams of charcoal produce 97
thousand calories, and 2 grams of hydrogen 69 thousand calories,
it follows that, if the hydrogen and carbon of the acetylene were
burnt there would be only 2 × 97 + 69, or 263 thousand calories
produced. It is evident, then, that acetylene in its formation
absorbs 310-263, or 47 thousand calories.

For considerations relative to the combustion of carbon
compounds, we will first enumerate the quantity of heat separated
by the combustion of definite chemical carbon compounds, and then
give a few figures bearing on the kinds of fuel used in practice.

For molecular quantities in perfect combustion the following
amounts of heat are given out (when gaseous carbonic anhydride
and liquid water are formed), according to Thomsen's data (1)
for gaseous C_{_n_}H_{2_n_ + 2}: 52·8 + 158·8_n_ thousand
calories; (2) for C_{_n_}H_{2_n_}: 17·7 + 158·1_n_ thousand
calories; (3) according to Stohmann (1888) for liquid saturated
alcohols, C_{_n_}H_{2_n_ + 2}O: 11·8 + 156·3_n_, and as the
latent heat of evaporation = about 8·2 + 0·6_n_, in a gaseous
state, 20·0 + 156·9_n_; (4) for monobasic saturated liquid
acids, C_{_n_}H_{2_n_}O_{2}:--95·3 + 154·3_n_, and as their
latent heat of evaporation is about 5·0 + 1·2_n_, in a gaseous
form, about--90 + 155_n_; (5) for solid saturated bibasic acids,
C_{_n_}H_{2_n_-2}O_{4}:--253·8 + 152·6_n_, if they are expressed
as C_{_n_}H_{2_n_}C_{2}H_{2}O_{4}, then 51·4 + 152·6_n_; (6)
for benzene and its liquid homologues (still according to
Stohmann) C_{_n_}H_{2_n_-6}:--158·6 + 156·3_n_, and in a gaseous
form about--155 + 157_n_; (7) for the gaseous homologues of
acetylene, C_{_n_}H_{2_n_-2} (according to Thomsen)--5 + 157_n_.
It is evident from the preceding figures that the group CH_{2},
or CH_{3} substituted for H, on burning gives out from 152 to
159 thousand calories. This is less than that given out by C +
H_{2}, which is 97 + 69 or 166 thousand; the reason for this
difference (it would be still greater if carbon were gaseous)
is the amount of heat separated during the formation of CH_{2}.
According to Stohmann, for dextroglucose, C_{6}H_{12}O_{6}, it
is 673·7; for common sugar, C_{12}H_{22}O_{11}, 1325·7; for
cellulose, C_{6}H_{10}O_{5}, 678·0; starch, 677·5; dextrin,
666·2; glycol, C_{2}H_{6}O_{2}, 281·7; glycerine, 397·2, &c.
The heat of combustion of the following solids (determined
by Stohmann) is expressed per unit of weight: naphthalene,
C_{10}H_{8}, 9,621; urea, CN_{2}H_{4}O, 2,465; white of egg,
5,579; dry rye bread, 4,421; wheaten bread, 4,302; tallow, 9,365;
butter, 9,192; linseed oil, 9,323. The most complete collection
of arithmetical data for the heats of combustion will be found
in V. F. Longinin's work, 'Description of the Various Methods
of Determining the Heats of Combustion of Organic Compounds'
(Moscow, 1894).

The number of units of heat given out by _unit weight_ during the
complete combustion and cooling of the following ordinary kinds
of fuel in their usual state of dryness and purity are:--(1) for
wood charcoal, anthracite, semi-anthracite, bituminous coal and
coke, from 7,200 to 8,200; (2) dry, long flaming coals, and the
best brown coals, from 6,200 to 6,800; (3) perfectly dry wood,
3,500; hardly dry, 2,500; (4) perfectly dry peat, best kind,
4,500; compressed and dried, 3,000; (5) petroleum refuse and
similar liquid hydrocarbons, about 11,000; (6) illuminating gas
of the ordinary composition (about 45 vols. H, 40 vols. CH_{4}, 5
vols. CO, and 5 vols. N), about 12,000; (7) producer gas (_see_
next Chapter), containing 2 vols. carbonic anhydride, 30 vols.
carbonic oxide, and 68 vols. nitrogen _for one part by weight of
the whole carbon burnt_, 5,300, and for one part by weight of the
gas, 910, units of heat; and (8) water gas (_see_ next chapter)
containing 4 vols. carbonic anhydride, 8 vols. N_{2}, 24 vols.
carbonic oxide, and 46 vols. H_{2}, for one part by weight of
the carbon consumed in the _generator_ 10,900, and for one part
by weight of the gas, 3,600 units of heat. In these figures,
as in all calorimetric observations, the water produced by the
combustion of the fuel is supposed to be liquid. As regards the
temperature reached by the fuel, it is important to remark that
for solid fuel it is indispensable to admit (to ensure complete
combustion) twice the amount of air required, but liquid, or
pulverised fuel, and especially gaseous fuel, does not require an
excess of air; therefore, a kilogram of charcoal, giving 8,000
units of heat, requires about 24 kilograms of air (3 kilograms
of air per thousand calories) and a kilogram of producer gas
requires only 0·77 kilogram of air (0·85 kilo. of air per 1,000
calories), 1 kilogram of water gas about 4·5 of air (1·25 kilo.
of air per 1,000 calories).

There is one substance known among the saturated hydrocarbons composed of 1 atom of carbon and 4 atoms of hydrogen; this is a compound containing the highest percentage of hydrogen (CH_{4} contains 25 per cent. of hydrogen), and at the same time it is the only hydrocarbon whose molecule contains but a single atom of carbon. This saturated hydrocarbon, CH_{4}, is called _marsh gas_ or _methane_. If vegetable or animal refuse suffers decomposition in a space where the air has not free access, or no access at all, then the decomposition is accompanied with the formation of marsh gas, and this either at the ordinary temperature, or at a comparatively much higher one. On this account _plants_, when decomposing under water in _marshes_, give out this gas.[29 bis] It is well known that if the mud in bogs be stirred up, the act is accompanied with the evolution of a large quantity of gas bubbles; these may, although slowly, also separate of their own accord. The gas which is evolved consists principally of marsh gas.[30] If wood, coal, or many other vegetable or animal substances are decomposed by the _action of heat_ without access of air--that is, are subjected to dry distillation--they, in addition to many other gaseous products of decomposition (carbonic anhydride, hydrogen, and various other substances), evolve a great deal of methane. Generally the gas which is used for lighting purposes is obtained by this means and therefore always contains marsh gas, mixed with dry hydrogen and other vapours and gases, although it is subsequently purified from many of them.[31] As the decomposition of the organic matter, which forms coal, is still going on underground, the evolution of large quantities of marsh gas frequently occurs in coal-mines.[32] When mixed with air it forms an explosive mixture, which forms one of the great dangers of coal mining, as subterranean work has always to be carried on by lamp-light. This danger is, however, overcome by the use of Humphry Davy's safety lamp.[33] Sir Humphry Davy observed that on introducing a piece of wire gauze into a flame, it absorbs so much heat that combustion does not proceed beyond it (the unburnt gases which pass through it may be ignited on the other side). In accordance with this, the flame of the Davy lamp is surrounded with a thick glass (as shown in the drawing), and has no communication whatever with the explosive mixture except through a wire gauze which prevents it igniting the mixture of the marsh-gas issuing from the coal with air. In some districts, particularly in those where petroleum is found--as, for instance, near Baku, where a temple of the Indian fire-worshippers was built, and in Pennsylvania, and other places--marsh gas in abundance issues from the earth, and it is used, like coal gas, for the purposes of lighting and warming.[34] Tolerably pure marsh gas[35] may be obtained by heating a mixture of an acetate with an alkali. Acetic acid, C_{2}H_{4}O_{2}, on being heated is decomposed into marsh gas and carbonic anhydride, C_{2}H_{4}O_{2} = CH_{4} + CO_{2}.

[29 bis] Manure which decomposes under the action of bacteria gives off
CO_{2} and CH_{4}.

[30] It is easy to collect the gas which is evolved in marshy places
if a glass bottle be inverted in the water and a funnel put into
it (both filled with water); if the mud of the bottom be now
agitated, the bubbles which rise may be easily caught by the
inverted funnel.

[31]

Illuminating gas is generally prepared by heating gas coal
(_see_ Note 6) in oval cylindrical horizontal cast-iron or clay
retorts. Several such retorts _BB_ (fig. 58) are disposed in the
furnace _A_, and heated together. When the retorts are heated
to a red heat, lumps of coal are thrown into them, and they
are then closed with a closely fitting cover. The illustration
shows the furnace, with five retorts. Coke (_see_ Note 1, dry
distillation) remains in the retorts, and the volatile products
in the form of vapours and gases travel along the pipe _d_,
rising from each retort. These pipes branch above the stove, and
communicate with the receiver _f_ (hydraulic main) placed above
the furnace. Those products of the dry distillation which most
easily pass from the gaseous into the liquid and solid states
collect in the hydraulic main. From the hydraulic main the
vapours and gases travel along the pipe _g_ and the series of
vertical pipes _j_ (which are sometimes cooled by water trickling
over the surface), where the vapours and gases cool from the
contact of the colder surface, and a fresh quantity of vapour
condenses. The condensed liquids pass from the pipes _g_ and _j_
and into the troughs _H_. These troughs always contain liquid
at a constant level (the excess flowing away) so that the gas
cannot escape, and thus they form, as it is termed, a hydraulic
joint. In the state in which it leaves the condensers the gas
consists principally of the following vapours and gases: (1)
vapour of water, (2) ammonium carbonate, (3) liquid hydrocarbons,
(4) hydrogen sulphide, H_{2}S, (5) carbonic anhydride, CO_{2},
(6) carbonic oxide, CO, (7) sulphurous anhydride, SO_{2}, but
a great part of the illuminating gas consists of (8) hydrogen,
(9) marsh gas, (10) olefiant gas, C_{2}H_{4}, and other gaseous
hydrocarbons. The hydrocarbons (3, 9, and 10), the hydrogen,
and carbonic oxide are capable of combustion, and are useful
component parts, but the carbonic anhydride, the hydrogen
sulphide, and sulphurous anhydride, as well as the vapours of
ammonium carbonate, form an injurious admixture, because they
do not burn (CO_{2}, SO_{2}) and lower the temperature and
brilliancy of the flame, or else, although capable of burning
(for example, H_{2}S, CS_{2}, and others), they give out during
combustion sulphurous anhydride which has a disagreeable smell,
is injurious when inhaled, and spoils many surrounding objects.
In order to separate the injurious products, the gas is washed
with water, a cylinder (not shown in the illustration) filled
with coke continually moistened with water serving for this
purpose. The water coming into contact with the gas dissolves
the ammonium carbonate; hydrogen sulphide, carbonic anhydride,
and sulphurous anhydride, being only partly soluble in water,
have to be got rid of by a special means. For this purpose the
gas is passed through moist lime or other alkaline liquid, as
the above-mentioned gases have acid properties and are therefore
retained by the alkali. In the case of lime, calcium carbonate,
sulphite and sulphide, all solid substances, are formed. It is
necessary to renew the purifying material as its absorbing power
decreases. A mixture of lime and sulphate of iron, FeSO_{4},
acts still better, because the latter, with lime, Ca(HO)_{2},
forms ferrous hydroxide, Fe(HO)_{2} and gypsum, CaSO_{4}. The
suboxide (partly turning into oxide) of iron absorbs H_{2}S,
forming FeS and H_{2}O, and the gypsum retains the remainder of
the ammonia, the excess of lime absorbing carbonic anhydride
and sulphuric anhydride. [In English works a native hydrated
ferric hydroxide is used for removing hydrogen sulphide.] This
purification of the gas takes place in the apparatus _L_, where
the gas passes through perforated trays _m_, covered with
sawdust mixed with lime and sulphate of iron. It is necessary
to remark that in the manufacture of gas it is indispensable
to draw off the vapours from the retorts, so that they should
not remain there long (otherwise the hydrocarbons would in a
considerable degree be resolved into charcoal and hydrogen),
and also to avoid a great pressure of gas in the apparatus,
otherwise a quantity of gas would escape at all cracks such as
must inevitably exist in such a complicated arrangement. For
this purpose there are special pumps (exhausters) so regulated
that they only pump off the quantity of gas formed (the pump is
not shown in the illustration). The purified gas passes through
the pipe _n_ into the gasometer (gasholder) _P_, a dome made of
iron plate. The edges of the dome dip into water poured into a
ring-shaped channel _g_, in which the sides of the dome rise and
fall. The gas is collected in this holder, and distributed to its
destination by pipes communicating with the pipe _o_, issuing
from the dome. The pressure of the dome on the gas enables it, on
issuing from a long pipe, to penetrate through the small aperture
of the burner. A hundred kilograms of coal give about 20 to 30
cubic metres of gas, having a density from four to nine times
greater than that of hydrogen. A cubic metre (1,000 litres) of
hydrogen weighs about 87 grams; therefore 100 kilograms of coal
give about 18 kilograms of gas, or about one-sixth of its weight.
Illuminating gas is generally lighter than marsh gas, as it
contains a considerable amount of hydrogen, and is only heavier
than marsh gas when it contains much of the heavier hydrocarbons.
Thus olefiant gas, C_{2}H_{4}, is fourteen times, and the
vapours of benzene thirty-nine times, heavier than hydrogen, and
illuminating gas sometimes contains 15 p.c. of its volume of
them. The brilliancy of the flame of the gas increases with the
quantity of olefiant gas and similar heavy hydrocarbons, as it
then contains more carbon for a given volume and a greater number
of carbon particles are separated. Gas usually contains from 35
to 60 p.c. of its volume of marsh gas, from 30 to 50 p.c. of
hydrogen, from 3 to 5 p.c. of carbonic oxide, from 2 to 10 p.c.
heavy hydrocarbons, and from 3 to 10 p.c. of nitrogen. Wood gives
almost the same sort of gas as coal and almost the same quantity,
but the wood gas contains a great deal of carbonic anhydride,
although on the other hand there is an almost complete absence
of sulphur compounds. Tar, oils, naphtha, and such materials
furnish a large quantity of good illuminating gas. An ordinary
burner of 8 to 12 candle-power burns 5 to 6 cubic feet of coal
gas per hour, but only 1 cubic foot of naphtha gas. One pood (36
lbs. Eng.) of naphtha gives 500 cubic feet of gas--that is, one
kilogram of naphtha produces about one cubic metre of gas. The
formation of combustible gas by heating coal was discovered in
the beginning of the last century, but only put into practice
towards the end by Le-Bon in France and Murdoch in England. In
England, Murdoch, together with the renowned Watt, built the
first gas works in 1805.

]

In practice illuminating gas is not only used for lighting
(electricity and kerosene are cheaper in Russia), but also as
the motive power for gas engines (_see_ p. 175), which consume
about half a cubic metre per horse-power per hour; gas is also
used in laboratories for heating purposes. When it is necessary
to concentrate the heat, either the ordinary blowpipe (fig. 59)
is applied, placing the end in the flame and blowing through
the mouthpiece; or, in other forms, gas is passed through the
blowpipe; when a large, hot, smokeless flame is required for
heating crucibles or glass-blowing, a foot-blower is used.
High temperatures, which are often required for laboratory and
manufacturing purposes, are most easily attained by the use of
gaseous fuel (illuminating gas, producer gas, and water gas,
which will be treated of in the following chapter), because
complete combustion may be effected without an access of air. It
is evident that in order to obtain high temperatures means must
be taken to diminish the loss of heat by radiation, and to ensure
perfect combustion.

[32] The gas which is set free in coal mines contains a good deal
of nitrogen, some carbonic anhydride, and a large quantity of
marsh gas. The best means of avoiding an explosion consists
in efficient ventilation. It is best to light coal mines with
electric lamps.

[33] The Davy lamp, of which an improved form is represented in the
accompanying figure, is used for lighting coal and other mines
where combustible gas is found. The wick of the lamp is enclosed
in a thick glass cylinder which is firmly held in a metallic
holder. Over this a metallic cylinder and the wire gauze are
placed. The products of combustion pass through the gauze, and
the air enters through the space between the cylinder and the
wire gauze. To ensure greater safety the lamp cannot be opened
without extinguishing the flame.

[34] In Pennsylvania (beyond the Alleghany mountains) many of the
shafts sunk for petroleum only emitted gas, but many useful
applications for it were found and it was conducted in metallic
pipes to works hundreds of miles distant, principally for
metallurgical purposes.

[35] The purest gas is prepared by mixing the liquid substance called
zinc methyl, Zn(CH_{3})_{2}, with water, when the following
reaction occurs:

Zn(CH_{3})_{2} + 2HOH = Zn(HO)_{2} + 2CH_{3}H.

An alkali--for instance, NaHO--gives with acetic acid a salt, C_{2}H_{3}NaO_{2}, which on decomposition retains carbonic anhydride, forming a carbonate, Na_{2}CO_{3}, and marsh gas is given off:

C_{2}H_{3}NaO_{2} + NaHO = Na_{2}CO_{3} + CH_{4}

Marsh gas is difficult to liquefy; it is almost insoluble in water, and is without taste or smell. The most important point in connection with its chemical reactions is that it does not combine directly with anything, whilst the other hydrocarbons which contain less hydrogen than expressed by the formula C_{_n_}H_{2_n_ + 2} are capable of combining with hydrogen, chlorine, certain acids, &c.

If the law of substitution gives a very simple explanation of the formation of hydrogen peroxide as a compound containing two aqueous residues (OH)(OH), then on the basis of this law all hydrocarbons ought to be derived from methane, CH_{4}, as being the simplest hydrocarbon.[36] The increase in complexity of a molecule of methane is brought about by the faculty of mutual combination which exists in the atoms of carbon, and, as a consequence of the most detailed study of the subject, much that might have been foreseen and conjectured from the law of substitution has been actually brought about in such a manner as might have been predicted, and although this subject on account of its magnitude really belongs, as has been already stated, to the sphere of organic chemistry, it has been alluded to here in order to show, although only in part, the best investigated example of the application of the law of substitution. According to this law, a molecule of methane, CH_{4}, is capable of undergoing substitution in the four following ways:--(1) Methyl substitution, when the radicle, equivalent to hydrogen, called _methyl_ CH_{3}, replaces hydrogen. In CH_{4} this radicle is combined with H and therefore can replace it, as (OH) replaces H because with it it gives water; (2) methylene substitution, or the exchange between H_{2} and CH_{2} (this radicle is called methylene), is founded on a similar division of the molecule CH_{4} into two equivalent parts, H_{2} and CH_{2}; (3) acetylene substitution, or the exchange between CH on the one hand and H_{3} on the other; and (4) carbon substitution--that is, the substitution of H_{4} by an atom of carbon C, which is founded on the law of substitution just as is the methyl substitution. These four cases of substitution render it possible to understand the principal relations of the hydrocarbons. For instance, the _law of even numbers_ is seen from the fact that in all the cases of substitution mentioned the hydrogen atoms increase or decrease by an even number; but as in CH_{4} they are likewise even, it follows that no matter how many substitutions are effected there will always be obtained an even number of hydrogen atoms. When H is replaced by CH_{3} there is an increase of CH_{2}; when H_{2} is replaced by CH_{2} there is no increase of hydrogen; in the acetylene substitution CH replaces H_{3}, therefore there is an increase of C and a decrease of H_{2}; in the carbon substitution there is a decrease of H_{4}. In a similar way the _law of limit_ may be deduced as a corollary of the law of substitution. For the largest possible quantity of hydrogen is introduced by the methyl substitution, since it leads to the addition of CH_{2}; starting from CH_{4} we obtain C_{2}H_{6}, C_{3}H_{8}, and in general, C_{_n_}H_{2_n_+2}, and these contain the greatest possible amount of hydrogen. Unsaturated hydrocarbons, containing less hydrogen, are evidently only formed when the increase of the new molecule derived from methane proceeds from one of the other forms of substitution. When the methyl substitution alone takes place in methane, CH_{4}, it is evident that the saturated hydrocarbon formed is C_{2}H_{6} or (CH_{3})(CH_{3}).[37] This is called _ethane_. By means of the methylene substitution alone, _ethylene_, C_{2}H_{4}, or (CH_{2})(CH_{2}) may be directly obtained from CH_{4}, and by the acetylene substitution C_{2}H_{2} or (CH)(CH), or _acetylene_, both the latter being unsaturated hydrocarbons. Thus we have all the possible hydrocarbons with two atoms of carbon in the molecule, C_{2}H_{6}, ethane, C_{2}H_{4}, ethylene, and C_{2}H_{2}, acetylene. But in them, according to the law of substitution, the same forms of substitution may be repeated--that is, the methyl, methylene, acetylene, and even carbon substitutions (because C_{2}H_{6} will still contain hydrogen when C replaces H_{4}) and therefore further substitutions will serve as a source for the production of a fresh series of saturated and unsaturated hydrocarbons, containing more and more carbon in the molecule and, in the case of the acetylene substitution and carbon substitution, containing less and less hydrogen. Thus _by means of the law of substitution we can foresee_ not only the limit C_{_n_}H_{2_n_+2}, but an unlimited number of unsaturated hydrocarbons, C_{_n_}H_{2_n_}, C_{_n_}H_{2_n_-2} ... C_{_n_}H_{2(_n-m_)}, where _m_ varies from 0 to _n_-1,[38] and where _n_ increases indefinitely. From these facts not only does the existence of a multitude of polymeric hydrocarbons, differing in molecular weight, become intelligible, but it is also seen that there is a possibility of cases of isomerism with the same molecular weight. This _polymerism_ so common to hydrocarbon compounds is already apparent in the first unsaturated series C_{_n_}H_{2_n_}, because all the terms of this series C_{2}H_{4}, C_{3}H_{6}, C_{4}H_{8} ... C_{30}H_{60} ... have one and the same composition CH_{2}, but different molecular weights, as has been already explained in Chapter VII. The differences in the vapour density, boiling points, and melting points, of the quantities entering into reactions,[39] and the methods of preparation[40] also so clearly tally with the conception of polymerism, that this example will always be the clearest and most conclusive for the illustration of polymerism and molecular weight. Such a case is also met with among other hydrocarbons. Thus benzene, C_{6}H_{6}, and cinnamene, C_{8}H_{8}, correspond with the composition of acetylene or to a compound of the composition CH.[41] The first boils at 81°, the second at 144°; the specific gravity of the first is 0·899; that of the second, 0·925, at 0°--that is, here also the boiling point rises with the increase of molecular weight, and so also, as might be expected, does the density.

[36] Methylene, CH_{2}, does not exist. When attempts are made to
obtain it (for example, by removing X_{2} from CH_{2}X_{2}),
C_{2}H_{4} or C_{3}H_{6} are produced--that is to say, it
undergoes polymerisation.

[37] Although the methods of formation and the reactions connected with
hydrocarbons are not described in this work, because they are
dealt with in organic chemistry, yet in order to clearly show
the mechanism of those transformations by which the carbon
atoms are built up into the molecules of the carbon compounds,
we here give a general example of reactions of this kind. From
marsh gas, CH_{4}, on the one hand the substitution of chlorine
or iodine, CH_{3}Cl, CH_{3}I, for the hydrogen may be effected,
and on the other hand such metals as sodium may be substituted
for the hydrogen, _e.g._ CH_{3}Na. These and similar products of
substitution serve as a means of obtaining other more complex
substances from given carbon compounds. If we place the two
above-named products of substitution of marsh gas (metallic and
haloid) in mutual contact, the metal combines with the halogen,
forming a very stable compound--namely, common salt, NaCl, and
the carbon groups which were in combination with them separate in
mutual combination, as shown by the equation:

CH_{3}Cl + CH_{3}Na = NaCl + C_{2}H_{6}.

This is the most simple example of the formation of a complex
hydrocarbon from these radicles. The cause of the reaction must
be sought for in the property which the haloid (chlorine) and
sodium have of entering into mutual combination.

[38] When _m_ = _n_-1, we have the series C_{_n_}H_{2}. The lowest
member is acetylene, C_{2}H_{2}. These are hydrocarbons
containing a minimum amount of hydrogen.

[39] For instance, ethylene, C_{2}H_{4}, combines with Br_{2}, HI,
H_{2}SO_{4}, as a whole molecule, as also does amylene,
C_{5}H_{10}, and, in general, C_{_n_}H_{2_n_}.

[40] For instance, ethylene is obtained by removing the water from
ethyl alcohol, C_{2}H_{5}(OH), and amylene, C_{5}H_{10}, from
amyl alcohol, C_{5}H_{11}(OH), or in general C_{_n_}H_{2_n_},
from C_{_n_}H_{2_n_+1}(OH).

[41] Acetylene and its polymerides have an empirical composition CH,
ethylene and its homologues (and polymerides) CH_{2}, ethane
CH_{3}, methane CH_{4}. This series presents a good example of
the law of multiple proportions, but such diverse proportions
are met with between the number of atoms of the carbon and
hydrogen in the hydrocarbons already known that the accuracy of
Dalton's law might be doubted. Thus the substances C_{30}H_{62}
and C_{30}H_{60} differ so slightly in their composition by
weight as to be within the limits of experimental error, but
their reactions and properties are so distinct that they can be
distinguished beyond a doubt. Without Dalton's law chemistry
could not have been brought to its present condition, but it
cannot alone express all those gradations which are quite clearly
understood and predicted by the law of Avogadro-Gerhardt.