Chapter XX: Sulphur, Selenium, and Tellurium (3)

[42] Thus when alkali waste, which contains calcium sulphide, undergoes
oxidation in the air it first forms a calcium polysulphide, and
then calcium thiosulphate, CaS_{2}O_{3}. When iron or zinc acts on
a solution of sulphurous acid, besides the hyposulphurous acid
first formed, a mixture of sulphite and thiosulphate is obtained
(Note 39), 3SO_{2} + Zn_{2} = ZnSO_{3} + ZnS_{2}O_{3}. In this
case, as in the formation of hyposulphurous acid, there is no
hydrogen liberated. One of the most common methods for preparing
thiosulphates consists in the _action of sulphur on the alkalis_.
The reaction is accomplished by the formation of sulphides and
thiosulphates, just as the reaction of chlorine on alkalis is
accompanied by the formation of hypochlorites and chlorides; hence
in this respect the thiosulphates hold the same position in the
order of the compounds of sulphur as the hypochlorites do among
the chlorine compounds. The reaction of caustic soda on an excess
of sulphur may be expressed thus: 6NaHO + 12S = 2Na_{2}S_{5} +
Na_{2}S_{2}O_{3} + 3H_{2}O. Thus sulphur is soluble in alkalis. On
a large scale sodium thiosulphate, Na_{2}S_{2}O_{3}, is prepared
by first heating sodium sulphate with charcoal, to form sodium
sulphide, which is then dissolved in water and treated with
sulphurous anhydride. The reaction is complete when the solution
has become slightly acid. A certain amount of caustic alkali is
added to the slightly acid solution; a portion of the sulphur is
thus precipitated, and the solution is then boiled and evaporated
when the salt crystallises out. The saturation of the solution of
sodium sulphide by sulphurous anhydride is carried on in different
ways--for example, by means of coke-towers, by causing the
solution of sulphide to trickle over the coke, and the sulphurous
anhydride, obtained by burning sulphur, to pass up the coke-tower
from below. An excess of sulphurous anhydride must be avoided, as
otherwise sodium trithionate is formed. Sodium thiophosphate is
also prepared by the double decomposition of the soluble calcium
thiosulphate with sodium sulphate or carbonate, in which case
calcium sulphate or carbonate is precipitated. The calcium
thiosulphate is prepared by the action of sulphurous anhydride on
either calcium sulphide or alkali waste. A dilute solution of
calcium thiosulphate may be obtained by treating alkali waste
which has been exposed to the action of air with water. On
evaporation, this solution gives crystals of the salt containing
CaS_{2}O_{3},5H_{2}O. A solution of calcium thiosulphate must be
evaporated with great care, because otherwise the salt breaks up
into sulphur and calcium sulphide. Even the crystallised salt
sometimes undergoes this change.

The crystals of sodium thiosulphate are stable, do not effloresce
and at 0° dissolve in one part of water, and at 20° in 0·6 part.
The solution of this salt does not undergo any change when boiled
for a short time, but after prolonged boiling it deposits sulphur.
The crystals fuse at 56°, and lose all their water at 100°. When
the dry salt is ignited it gives sodium sulphide and sulphate.
With acids, a solution of the thiosulphate soon becomes cloudy and
deposits an exceedingly fine powder of sulphur (Note 10). If the
amount of acid added be considerable, it also evolves sulphurous
anhydride: H_{2}S_{2}O_{3} = H_{2}O + S + SO_{2}. Sodium
thiosulphate has many practical uses; it is used in photography
for dissolving silver chloride and bromide. Its solvent action on
silver chloride may be taken advantage of in extracting this metal
as chloride from its ores. In dissolving, it forms a double salt
of silver and sodium: AgCl + Na_{2}S_{2}O_{3} = NaCl +
AgNaS_{2}O_{3}. Sodium thiosulphate is an _antichlor_--that is, a
substance which hinders the destructive action of free chlorine
owing to its being very easily oxidised by chlorine into sulphuric
acid and sodium chloride. The reaction with iodine is different,
and is remarkable for the accuracy with which it proceeds. The
iodine takes up half the sodium from the salt and converts it into
a tetrathionate; 2Na_{2}S_{2}O_{3} + I_{2} = 2NaI +
Na_{2}S_{4}O_{6}, and hence this reaction is employed for the
determination of free iodine. As iodine is expelled from potassium
iodide by chlorine, it is possible also to determine the amount of
chlorine by this method if potassium iodide be added to a solution
containing chlorine. And as many of the higher oxides are able to
evolve iodine from potassium iodide, or chlorine from hydrochloric
acid (for example, the higher oxides of manganese, chromium, &c.),
it is also possible to determine the amounts of these higher
oxides by means of sodium thiosulphate and liberated iodine. This
forms the basis of the iodometric method of volumetric analysis.
The details of these methods will be found in works on analytical
chemistry.

On adding a solution of a _lead salt_ gradually to a solution of
sodium thiosulphate a white precipitate of lead thiosulphate,
PbS_{2}O_{3}, is formed (a soluble double salt is first formed,
and if the action be rapid, lead sulphide). When this substance is
heated at 200°, it undergoes a change and takes fire. Sodium
thiosulphate in solution rapidly reduces cupric salts to cuprous
salts by means of the sulphurous acid contained in the
thiosulphate, but the resultant cuprous oxide is not precipitated,
because it passes into the state of a thiosulphate and forms a
double salt. These double cuprous salts are excellent reducing
agents. The solution when heated gives a black precipitate of
copper sulphide.

The following formulæ sufficiently explain the position held by
thiosulphuric acid among the other acids of sulphur:

Sulphurous acid SO_{2}H(OH)
Sulphuric acid SO_{2}OH(OH)
Thiosulphuric acid SO_{2}SH(OH)
Hyposulphurous acid SO_{2}H(SO_{2}H)
Dithionic acid SO_{2}OH(SO_{2}OH)

At one time it was thought that all the salts of thiosulphuric
acid only existed in combination with water, and it was then
supposed that their composition was H_{4}S_{2}O_{4}, or
H_{2}SO_{2}, but Popp obtained the anhydrous salts.

Although sulphur, oxidising at a high temperature, only forms a small quantity of sulphuric anhydride, SO_{3}, and nearly all passes into sulphurous anhydride, still the latter may be converted into the higher oxide, or _sulphuric anhydride_, SO_{3}, by many methods. Sulphuric anhydride is a solid crystalline substance at the ordinary temperature; it is easily fusible (15°), and volatile (46°), and rapidly attracts moisture. Although it is formed by the combination of sulphurous anhydride with oxygen, it is capable of further combination. Thus it combines with water, hydrochloric acid, ammonia, with many hydrocarbons, and even with sulphuric acid, boric and nitrous anhydrides, &c., and also with bases which burn directly in its vapour, forming sulphates in the presence of traces of moisture (_see_ Chapter IX., Note 29). The oxidation of sulphurous anhydride, SO_{2}, into sulphuric anhydride, SO_{3}, is effected by passing a mixture of the former and dry oxygen or air over incandescent spongy platinum. An increase of pressure accelerates the reaction (Hanisch). If the product be passed into a cold vessel, crystalline sulphuric anhydride is deposited upon the sides of the vessel, but as it is difficult to avoid all traces of moisture it always contains compounds of its hydrates: H_{2}S_{2}O_{7} and H_{2}S_{4}O_{13}, whose presence so modifies the properties of the anhydride (Weber) that formerly two modifications of the anhydride were recognised. The same sulphuric anhydride may be obtained from certain anhydrous sulphates, or those which are almost so, which are decomposed by heat, whilst an impure but perfectly anhydrous anhydride is formed by distillation over phosphoric anhydride. For instance, acid sodium sulphate, NaHSO_{4}, and the pyro- or di-sulphate, Na_{2}S_{2}O_{7} (Chapter XII.) formed from it, when ignited evolve sulphuric anhydride. Green vitriol--that is, ferrous sulphate, FeSO_{4}--belongs to the number of those sulphates which easily give off sulphuric anhydride under the action of heat. It contains water of crystallisation and parts with it when it is heated, but the last equivalent of water is driven off with difficulty, just as is the case with magnesium sulphate, MgSO_{4}7H_{2}O; however, when thoroughly heated, this evolution of sulphuric anhydride does take place, although not completely, because at a high temperature a portion of it is decomposed by the ferrous oxide (SO_{3} + 2FeO), which is converted into ferric oxide, Fe_{2}O_{3}, and in consequence part of the sulphuric anhydride is converted into sulphurous anhydride. Thus the products of the decomposition of ferrous sulphate will be: ferric oxide, Fe_{2}O_{3}, sulphurous anhydride, SO_{2}, and sulphuric anhydride, SO_{3}, according to the equation: 2FeSO_{4} = Fe_{2}O_{3} + SO_{2} + SO_{3}. As water still remains with the ferrous sulphate when it is heated, the result will partially consist of the hydrate H_{2}SO_{4}, with anhydride, SO_{3}, dissolved in it. Sulphuric acid was for a long time prepared in this manner; the process was formerly carried on on a large scale in the neighbourhood of Nordhausen, and hence the sulphuric acid prepared from ferrous sulphate is called _fuming Nordhausen acid_. At the present time the fuming acid is prepared by passing the volatile products of the decomposition of ferrous sulphate through strong sulphuric acid prepared by the ordinary method. The sulphurous anhydride is insoluble in it, but it absorbs the sulphuric anhydride. Sulphuric anhydride may be prepared not only by igniting FeSO_{4} or sodium pyrosulphate, Na_{2}S_{2}O_{7} (the decomposition proceeds at 600°), but also by heating a mixture of the latter and MgSO_{4} (Walters); in the former case a stable double salt MgNa_{2}(SO_{4})_{2} finally remains. It is also obtained by the direct combination of SO_{2} and O under the action of spongy platinum or asbestos coated with platinum black (C. Winkler's process). Nordhausen sulphuric acid fumes in air, owing to its containing and easily giving off sulphuric anhydride, and it is therefore also called _fuming sulphuric acid_; these fumes are nothing but the vapour of sulphuric anhydride combining with the moisture in the air and forming non-volatile sulphuric acid (hydrate).[43]

[43] Nordhausen sulphuric acid may serve as a very simple means for the
preparation of sulphuric anhydride. For this purpose the
Nordhausen acid is heated in a glass retort, whose neck is firmly
fixed in the mouth of a well-cooled flask. The access of moisture
is prevented by connecting the receiver with a drying-tube. On
heating the retort the vapours of sulphuric anhydride will pass
over into the receiver, where they condense; the crystals of
anhydride thus prepared will, however, contain traces of sulphuric
acid--that is, of the hydrate. By repeatedly distilling over
phosphoric anhydride, it is possible to obtain the pure anhydride,
SO_{3}, especially if the process be carried on without access of
air in a closed vessel.

The ordinary sulphuric anhydride, which is imperfectly freed from
the hydrate, is a snow-white, exceedingly volatile substance,
which crystallises (generally by sublimation) in long silky
prisms, and only gives the pure anhydride when carefully distilled
over P_{2}O_{5}. Freshly prepared crystals of almost pure
anhydride fuse at 16° into a colourless liquid having a specific
gravity at 26° = 1·91, and at 47° = 1·81; it volatilises at 46°.
After being kept for some time the anhydride, even containing only
small traces of water, undergoes a change of the following nature:
A small quantity of sulphuric acid combines by degrees with a
large proportion of the anhydride, forming polysulphuric acids,
H_{2}SO_{4},_n_SO_{3}, which fuse with difficulty (even at 100°,
Marignac), but decompose when heated. In the entire absence of
water this rise in the fusing point does not occur (Weber), and
then the anhydride long remains liquid, and solidifies at about
+15°, volatilises at 40°, and has a specific gravity 1·94 at 16°.
We may add that Weber (1881), by treating sulphuric anhydride with
sulphur, obtained a blue lower oxide of sulphur, S_{2}O_{3}.
Selenium and tellurium also give similar products with SO_{3},
SeSO_{3}, and TeSO_{3}. Water does not act upon them.

Nordhausen sulphuric acid contains a peculiar compound of SO_{3} and H_{2}SO_{4}, or _pyrosulphuric acid_; an imperfect anhydride of sulphuric acid, H_{2}S_{2}O_{7}, analogous in composition with the salts Na_{2}S_{2}O_{7}, K_{2}Cr_{2}O_{7}, and bearing the same relation to H_{2}SO_{4} that pyrophosphoric acid does to H_{3}PO_{4}. The bond holding the sulphuric acid and anhydride together is unstable. This is obvious from the fact that the anhydride may easily be separated from this compound, by the action of heat. In order to obtain the definite compound, the Nordhausen acid is cooled to 5°, or, better still, a portion of it is distilled until all the anhydride and a certain amount of sulphuric acid have passed over into the distillate, which will then solidify at the ordinary temperature, because the compound H_{2}SO_{4},SO_{3} fuses at 35°. Although this substance reacts on water, bases, &c., like a mixture of SO_{3} + H_{2}SO_{4}, still since a definite compound, H_{2}S_{2}O_{7}, exists in a free state and gives salts and a chloranhydride, S_{2}O_{5}Cl_{2},[44] we must admit the existence of a definite pyrosulphuric acid, like pyrophosphoric acid, only that the latter has a far greater stability and is not even converted into a perfect hydrate by water. Further, the salts M_{2}S_{2}O_{7} dissolved in water react in the same manner as the acid salts MHSO_{4}, whilst the imperfect hydrates of phosphoric acid (for example, PHO_{3}, H_{4}P_{2}O_{7}) have independent reactions even in an aqueous solution which distinguish them and their salts from the perfect hydrates.

[44] Pyrosulphuric chloranhydride, or _pyrosulphuryl chloride_,
S_{2}O_{5}Cl_{2}, corresponds to pyrosulphuric acid, in the same
way that sulphuryl chloride, SO_{2}Cl_{2}, corresponds to
sulphuric acid. The composition S_{2}O_{5}Cl_{2} = SO_{2}Cl_{2} +
SO_{3}. It is obtained by the action of the vapour of sulphuric
anhydride on sulphur chloride: S_{2}Cl_{2} + 5SO_{3} = 5SO_{2} +
S_{2}O_{5}Cl_{2}. It is also formed (and not sulphuryl chloride,
SO_{2}Cl_{2}, Michaelis) by the action of phosphorus pentachloride
in excess on sulphuric acid (or its first chloranhydride,
SHO_{3}Cl). It is an oily liquid, boiling at about 150°, and of
sp. gr. 1·8. According to Konovaloff (Chapter VII.), its vapour
density is normal. It should be noticed that the same substance is
obtained by the action of sulphuric anhydride on sulphur
tetrachloride, and also on carbon tetrachloride, and this
substance is the last product of the metalepsis of CH_{4}, and
therefore the comparison of SCl_{2} and S_{2}Cl_{2} with products
of metalepsis (_see_ later) also finds confirmation in particular
reactions. Rose, who obtained pyrosulphuryl chloride,
S_{2}O_{5}Cl_{2}, regarded it as SCl_{6},5SO_{3}, for at that time
an endeavour was always made to find two component parts of
opposite polarity, and this substance was cited as a proof of the
existence of a hexachloride, SCl_{6}. Pyrosulphuryl chloride is
decomposed by cold water, but more slowly than chlorosulphuric
acid and the other chloranhydrides.

The relation between pyrosulphuric acid and the normal acid will
be obvious if we express the latter by the formula OH(SO_{3}H),
because the sulphonic group (SO_{3}H) is then evidently equivalent
to OH, and consequently to H, and if we replace both the hydrogens
in water by this radicle we shall obtain (SO_{3}H)_{2}O--that is,
pyrosulphuric acid.

_Sulphuric acid_, H_{2}SO_{4}, is formed by the combination of its anhydride, SO_{3}, and water, with the evolution of a large amount of heat; the reaction SO_{3} + H_{2}O develops 21,300 heat units. The method of its preparation on a large scale, and most of the methods employed for its formation, are dependent on the oxidation of sulphurous anhydride, and the formation of sulphuric anhydride, which forms sulphuric acid under the action of water. The technical method of its manufacture has been described in Chapter VI. The acid obtained _from the lead chambers_ contains a considerable amount of water, and is also impure owing to the presence of oxides of nitrogen, lead compounds, and certain impurities from the burnt sulphur which have come over in a gaseous and vaporous state (for example, arsenic compounds). For practical purposes, hardly any notice is taken of the majority of these impurities, because they do not interfere with its general qualities. Most frequently endeavours are only made to remove, as far as possible, all the water which can be expelled.[45] That is, the object is to obtain the hydrate, H_{2}SO_{4}, from the dilute acid (60 per cent.), and this is effected by evaporation by means of heat. Every given mixture of water and sulphuric acid begins to part with a certain amount of aqueous vapour when heated to a certain definite temperature. At a low temperature either there is no evaporation of water, or there can even be an absorption of moisture from the air. As the removal of the water proceeds, the vapour tension of the residue decreases for the same temperature, and therefore the more dilute the acid the lower the temperature at which it gives up a portion of its water. In consequence of this, the removal of water from dilute solutions of sulphuric acid may be easily carried on (up to 75 p.c. H_{2}SO_{4}) in lead vessels, because at low temperatures dilute sulphuric acid does not attack lead. But as the acid becomes more concentrated the temperature at which the water comes over becomes higher and higher, and then the acid begins to act on lead (with the evolution of sulphuretted hydrogen and conversion of the lead into sulphate), and therefore lead vessels cannot be employed for the complete removal of the water. For this purpose the evaporation is generally carried on in glass or platinum retorts, like those depicted in figs. 87 and 88.

[45] The removal of the water, or concentration to almost the real
acid, H_{2}SO_{4}, is effected for two reasons: in the first place
to avoid the expense of transit (it is cheaper to remove the water
than to pay for its transit), and in the second place because many
processes--for instance, the refining of petroleum--require a
strong acid free from an excess of water, the weak acid having no
action. When in the manufacture of chamber acid, both the
Gay-Lussac tower (cold, situated at the end of the chambers) and
the Glover tower (hot, situated at the beginning of the plant,
between the chambers and ovens for the production of SO_{2}) are
employed, a mixture of nitrose (_i.e._ the product of the
Gay-Lussac tower) and chamber acid containing about 60 p.c.
H_{2}SO_{4}, is poured into the Glover tower, where under the
action of the hot furnace gases containing SO_{2}, and the water
held in the chamber acid (1) N_{2}O_{3} is evolved from the
nitrose; (2) water is expelled from the chamber acid; (3) a
portion of the SO_{2} is converted into H_{2}SO_{4}; and (4) the
furnace gases are cooled. Thus, amongst other things, the Glover
tower facilitates the concentration of the chamber acid (removal
of H_{2}O), but the product generally contains many impurities.

_The concentration of sulphuric acid_ in glass retorts is not a continuous process, and consists of heating the dilute 75 per cent. acid until it ceases to give off aqueous vapour, and until acid containing 93-98 per cent. H_{2}SO_{4} (66° Baumé) is obtained--and this takes place when the temperature reaches 320° and the density of the residue reaches 1·847 (66° Baumé).[46] The platinum vessels designed for the continuous concentration of sulphuric acid consist of a still _B_, furnished with a still head _E_, a connecting pipe _E F_, and a syphon tube _H R_, which draws off the sulphuric acid concentrated in the boiler. A stream of sulphuric acid previously concentrated in lead retorts to a density of about 60° Baumé--_i.e._ to 75 per cent. or a sp. gr. of 1·7--runs continuously into the retort through a syphon funnel _E´_. The apparatus is fed from above, because the acid freshly supplied is lighter than that which has already lost water, and also because the water is more easily evaporated from the freshly supplied acid at the surface. The platinum retort is heated, and the steam coming off[47] is condensed in a worm _F G_, whilst as fresh dilute acid is supplied to the boiler the acid already concentrated is drawn off through the syphon tube _H B_, which is furnished with a regulating cock by means of which the outflow of the concentrated acid from the bottom of the retort can be so regulated that it will always present one and the same specific gravity, corresponding with the strength required. For this purpose the acid flowing from the syphon is collected in a receiver _R_, in which a hydrometer, indicating its density, floats; if its density be less than 66° Baumé, the regulating cock is closed sufficiently to retard the outflow of sulphuric acid, so as to lengthen the time of its evaporation in the retort.[48]

[46] The difficulty with which the last portions of water are removed
is seen from the fact that the boiling becomes very irregular,
totally ceasing at one moment, then suddenly starting again, with
the rapid formation of a considerable amount of steam, and at the
same time bumping and even overturning the vessel in which it is
held. Hence it is not a rare occurrence for the glass retorts to
break during the distillation; this causes platinum retorts to be
preferred, as the boiling then proceeds quite uniformly.

[47] According to Regnault, the vapour tensions (in millimetres of
mercury) of the water given off by the hydrates of sulphuric acid,
H_{2}SO_{4},_n_H_{2}O, are--

_t_=5° 15° 30°

_n_ = 1 0·1 0·1 0·2
2 0·4 0·7 1·5
3 0·9 1·6 4·1
4 1·3 2·8 7·0
5 2·1 4·2 10·7
7 3·2 6·2 15·6
9 4·1 8·0 19·6
11 4·4 9·0 22·2
17 5·5 10·6 26·1

According to Lunge, the vapour tension of the aqueous vapour given
off from solutions of sulphuric acid containing _p_ per cent.
H_{2}SO_{4}, at _t_°, equals the barometric pressure 720 to 730
mm.

_p_= 10 20 30 40 50 60 70 80 85 90 95

_t_= 102° 105° 108° 114° 124° 141° 170° 207° 233° 262° 295°

The latter figures give the temperature at which water is easily
expelled from solutions of sulphuric acid of different strengths.
But the evaporation begins sooner, and concentration may be
carried on at lower temperatures if a stream of air be passed
through the acid. Kessler's process is based upon this (Note 48).

[48] The greatest part of the sulphuric acid is used in the soda
manufacture, in the conversion of the common salt into sulphate.
For this purpose an acid having a density of 60° Baumé is amply
sufficient. Chamber acid has a density up to 1·57 = 50° to 51°
Baumé; it contains about 35 per cent. of water. About 15 per cent.
of this water can be removed in leaden stills, and nearly all the
remainder may be expelled in glass or platinum vessels. Acid of
66° Baumé, = 1·847, contains about 96 per cent. of the hydrate
H_{2}SO_{4}. The density falls with a greater or less proportion
of water, the maximum density corresponding with 97-1/2 per cent.
of the hydrate H_{2}SO_{4}. The concentration of H_{2}SO_{4} in
platinum retorts has the disadvantage that sulphuric acid, upwards
of 90 per cent. in strength, does corrode platinum, although but
slightly (a few grams per tens of tons of acid). The retorts
therefore require repairing, and the cost of the platinum exceeds
the price obtained for concentrating the acid from 90 per cent. to
98 per cent. (in factories the acid is not concentrated beyond
this by evaporation in the air). This inconvenience has lately
(1891, by Mathey) been eliminated by coating the inside of the
platinum retorts with a thin (0·1 to 0·02 mm.) layer of gold which
is 40 times less corroded by sulphuric acid than platinum. Négrier
(1890) carries on the distillation in porcelain dishes, Blond by
heating a thin platinum wire immersed in the acid by means of an
electric current, but the most promising method is that of Kessler
(1891), which consists in passing hot air over sulphuric acid
flowing in a thin stream in stone vessels, so that there is no
boiling but only evaporation at moderate temperatures: the
transference of the heat is direct (and not through the sides of
the vessels), which economises the fuel and prevents the
distilling vessels being damaged.

When, by evaporation of the water, sulphuric acid attains a
density of 66° Baumé (sp. gr. 1·84), it is impossible to
concentrate it further, because it then distils over unchanged.
_The distillation of sulphuric acid_ is not generally carried on
on a large scale, but forms a laboratory process, employed when
particularly pure acid is required. The distillation is effected
either in platinum retorts furnished with corresponding condensers
and receivers, or in glass retorts. In the latter case, great
caution is necessary, because the boiling of sulphuric acid itself
is accompanied by still more violent jerks and greater
irregularity than even the evaporation of the last portions of
water contained in the acid. If the glass retort which holds the
strong sulphuric acid to be distilled be heated directly from
below, it frequently jerks and breaks. For greater safety the
heating is not effected from below, but at the sides of the
retort. The evaporation then does not proceed in the whole mass,
but only from the upper portions of the liquid, and therefore goes
on much more quietly. The acid may be made to boil quietly also by
surrounding the retort with good conductors of heat--for example,
iron filings, or by immersing a bunch of platinum wires in the
acid, as the bubbles of sulphuric acid vapour then form on the
extremities of the wires.

Strictly speaking, _sulphuric acid is not volatile_, and at its so-called boiling-point it really decomposes into its anhydride and water; its boiling-point (338°) being nothing else but its temperature of decomposition. The products of this decomposition are substances boiling much below the temperature of the decomposition of sulphuric acid. This conclusion with regard to the process of the distillation of sulphuric acid may be deduced from Bineau's observations on the vapour-density of sulphuric acid. This density referred to hydrogen proved to be half that which sulphuric acid should have according to its molecular weight, H_{2}SO_{4}, in which case it should be 49, whilst the observed density was equal to 24·5. Besides which, Marignac showed that the first portions of the sulphuric acid distilling over contain less of the elements of water than the portion which remains behind, or which distils over towards the end. This is explained by the fact that on distillation the sulphuric acid is decomposed, but a portion of the water proceeding from its decomposition is retained by the remaining mass of sulphuric acid, and therefore at first a mixture of sulphuric acid and sulphuric anhydride--_i.e._ fuming sulphuric acid--is obtained in the distillate. It is possible by repeating the distillation several times and only collecting the first portions of the distillate, to obtain a distinctly fuming acid. To obtain the definite hydrate H_{2}SO_{4} it is necessary to refrigerate a highly concentrated acid, of as great a purity as possible, to which a small quantity of sulphuric anhydride has been previously added. Sulphuric acid containing a small quantity (a fraction of a per cent. by weight) of water only freezes at a very low temperature, while the pure normal acid, H_{2}SO_{4}, solidifies when it is cooled below 0°, and therefore the normal acid first crystallises out from the concentrated sulphuric acid. By repeating the refrigeration several times, and pouring off the unsolidified portion, it is possible to obtain a pure _normal hydrate_, H_{2}SO_{4}, which melts at 10°·4. Even at 40° it gives off distinct fumes--that is, it begins to evolve sulphuric anhydride, which volatilises, and therefore even in a dry atmosphere the hydrate H_{2}SO_{4} becomes weaker, until it contains 1-1/2 p.c. of water.[49]

[49] Thus it appears that so common, and apparently so stable, a
compound as sulphuric acid decomposes even at a low temperature
with separation of the anhydride, but this decomposition is
restricted by a limit, corresponding to the presence of about
1-1/2 p.c. of water, or to a composition of nearly
H_{2}O,12H_{2}SO_{4}.

Now there is no reason for thinking that this substance is a
definite compound; it is an equilibrated system which does not
decompose under ordinary circumstances below 338°. Dittmar carried
on the distillation under pressures varying between 30 and 2,140
millimetres (of mercury), and he found that the composition of the
residue hardly varies, and contains from 99·2 to 98·2 per cent. of
the normal hydrate, although at 30 mm. the temperature of
distillation is about 210° and at 2,140 mm. it is 382°.
Furthermore, it is a fact of practical importance that under a
pressure of two atmospheres the distillation of sulphuric acid
proceeds very quietly.

Sulphuric acid may be _purified_ from the majority of its
impurities by distillation, if the first and last portions of the
distillate be rejected. The first portions will contain the oxides
of nitrogen, hydrochloric acid, &c., and the last portions the
less volatile impurities. The oxides of nitrogen may be removed by
heating the acid with charcoal, which converts them into volatile
gases. Sulphuric acid may be freed from arsenic by heating it with
manganese dioxide and then distilling. This oxidises all the
arsenic into non-volatile arsenic acid. Without a preliminary
oxidation it would partially remain as volatile arsenious acid,
and might pass over into the distillate. The arsenic may also be
driven off by first reducing it to arsenious acid, and then
passing hydrochloric acid gas through the heated acid. It is then
converted into arsenious chloride, which volatilises.

In a concentrated form sulphuric acid is commercially known as _oil of vitriol_, because for a long time it was obtained from green vitriol and because it has an oily appearance and flows from one vessel into another in a thick and somewhat sluggish stream, like the majority of oily substances, and in this clearly differs from such liquids as water, spirit, ether, and the like, which exhibit a far greater mobility. Among its reactions the first to be remarked is its faculty for the formation of many compounds. We already know that it combines with its anhydride, and with the sulphates of the alkali metals; that it is soluble in water, with which it forms more or less stable compounds. Sulphuric acid, when mixed with water, develops a very considerable amount of heat.[50]

[50] The amount of heat developed by the mixture of sulphuric acid with
water is expressed in the diagram on p. 77, Volume I., by the
middle curve, whose abscissæ are the percentage amounts of acid
(H_{2}SO_{4}) in the resultant solution, and ordinates the number
of units of heat corresponding with the formation of 100 cubic
centimetres of the solution (at 18°). The calculations on which
the curve is designed are based on Thomsen's determinations, which
show that 98 grams or a molecular amount of sulphuric acid, in
combining with _m_ molecules of water (that is, with _m_=18 grams
of water), develop the following number of units of heat, R:--

_m_ = 1 2 3 5 9
R = 6379 9418 11137 13108 14952
_c_ = 0·432 0·470 0·500 0·576 0·701
T = 127° 149° 146° 121° 82°

_m_ = 19 49 100 200
R = 16256 16684 16859 17066
_c_ = 0·821 0·914 0·954 0·975
T = 145° 19° 9° 5°

_c_ stands for the specific heat of H_{2}SO_{4}_m_H_{2}O
(according to Marignac and Pfaundler), and T for the rise in
temperature which proceeds from the mixture of H_{2}SO_{4} with
_m_H_{2}O. The diagram shows that contraction and rise of
temperature proceed almost parallel with each other.

Besides the normal hydrate H_{2}SO_{4}, _another definite
hydrate_, H_{2}SO_{4},H_{2}O (84·48 per cent. of the normal
hydrate, and 15·52 per cent. of water) is known; it
crystallises[50 bis] extremely easily in large six-sided prisms,
which form above 0°--namely, at about +8°·5; when heated to 210°
it loses water.[51] If the hydrates H_{2}SO_{4} and
H_{2}SO_{4},H_{2}O exist at low temperatures as definite
crystalline compounds, and if pyrosulphuric acid,
H_{2}SO_{4}SO_{3}, has the same property, and if they all
decompose with more or less ease on a rise of temperature, with
the disengagement of either SO_{3} or H_{2}O, and in their
ordinary form present all the properties of simple solutions, it
follows that between sulphuric anhydride, SO_{3}, and water,
H_{2}O, there exists a consecutive series of homogeneous liquids
or solutions, among which we must distinguish _definite
compounds_, and therefore it is quite justifiable to look for
other definite compounds between SO_{3} and H_{2}O, beyond the
conditions for a change of state. In this respect we may be guided
by the variation of properties of any kind, proceeding
concurrently with a variation in the composition of a solution.

[50 bis] Pickering (1890) showed (_a_) that dilute solutions of
sulphuric acid containing up to H_{2}SO_{4} + 10H_{2}O deposit ice
(at -0°·12 when there is 2,000H_{2}O per H_{2}SO_{4}, at -0°·23
when there is 1,000H_{2}O, at -1°·04 when there is 200H_{2}O, at
-2°·12 when there is 100H_{2}O, at -4°·5 when there is 50H_{2}O,
at -15°·7 when there is 20H_{2}O, and at -61° when the composition
of the solution is H_{2}SO_{4} + 10H_{2}O); (_b_) that for higher
concentrations crystals separate out at a considerable degree of
cold, having the composition H_{2}SO_{4}4H_{2}O, which melt at
-24°·5, and if either water or H_{2}SO_{4} be added to this
compound the temperature of crystallisation falls, so that a
solution of the composition 12H_{2}SO_{4} + 100H_{2}O gives
crystals of the above hydrate at -70°, 15H_{2}SO_{4} + 100H_{2}O
at -47°, 30H_{2}SO_{4} + 100H_{2}O at -32°, 40H_{2}SO_{4} +
100H_{2}O at -52°; (_c_) that if the amount of H_{2}SO_{4} be
still greater, then a hydrate H_{2}SO_{4}H_{2}O separates out and
melts at +8°·5, while the addition of water or sulphuric acid to
it lowers the temperature of crystallisation so that the
crystallisation of H_{2}SO_{4}H_{2}O from a solution of the
composition H_{2}SO_{4} + 1·73H_{2}O takes place at -22°,
H_{2}SO_{4} + 1·5H_{2}O at -6°·5, H_{2}SO_{4} + 1·2H_{2}O at
+3°·7, H_{2}SO_{4} + 0·75H_{2}O at +2°·8, H_{2}SO_{4} + 0·5H_{2}O
at -16°; (_d_) that when there is less than 40H_{2}O per
100H_{2}SO_{4}, refrigeration separates out the normal hydrate
H_{2}SO_{4}, which melts at +10°·35, and that a solution of the
composition H_{2}SO_{4} + 0·35H_{2}O deposits crystals of this
hydrate at -34°, H_{2}SO_{4} + 0·1H_{2}O at -4°·1, H_{2}SO_{4} +
0·05H_{2}O at +4°·9, while fuming acid of the composition
H_{2}SO_{4} + 0·05SO_{3} deposits H_{2}SO_{4} at about +7°. Thus
the temperature of the separation of crystals clearly
distinguishes the above four regions of solutions, and in the
space between H_{2}SO_{4} + H_{2}O and +25H_{2}O a particular
hydrate H_{2}SO_{4}4H_{2}O separates out, discovered by Pickering,
the isolation of which deserves full attention and further
research. I may add here that the existence of a hydrate
H_{2}SO_{4}4H_{2}O was pointed out in my work, _The Investigation
of Aqueous Solutions_, p. 120 (1887), upon the basis that it has
at all temperatures a smaller value for the coefficient of
expansion _k_ in the formula S_{_t_} = S_{0}/(1 - _kt_) than the
adjacent (in composition) solutions of sulphuric acid. And for
solutions approximating to H_{2}SO_{4}10H_{2}O in their
composition, _k_ is constant at all temperatures (for more dilute
solutions the value of _k_ increases with _t_ and for more
concentrated solutions it decreases). This solution (with
10H_{2}O) forms the point of transition between more dilute
solutions which deposit ice (water) when refrigerated and those
which give crystals of H_{2}SO_{4}4H_{2}O. According to R. Pictet
(1894) the solution H_{2}SO_{4}10H_{2}O freezes at -88° (but no
reference is made as to what separates out), _i.e._ at a lower
temperature than all the other solutions of sulphuric acid.
However, in respect to these last researches of R. Pictet (for
88·88 p.c. H_{2}SO_{4} -55°, for H_{2}SO_{4}H_{2}O +3·5°, for
H_{2}SO_{4}2H_{2}O -70°, for H_{2}SO_{4}4H_{2}O -40°, &c.) it
should be remarked that they offer some quite improbable data; for
example, for H_{2}SO_{4}75H_{2}O they give the freezing point as
0°, for H_{2}SO_{4}300H_{2}O +4°·5, and even for
H_{2}SO_{4}1000H_{2}O +0°·5, although it is well known that a
small amount of sulphuric acid lowers the temperature of the
formation of ice. I have found by direct experiment that a frozen
solidified solution of H_{2}SO_{4} + 300H_{2}O melted completely
at 0°.

[51] With an excess of snow, the hydrate H_{2}SO_{4},H_{2}O, like the
normal hydrate, gives a freezing mixture, owing to the absorption
of a large amount of heat (the latent heat of fusion). In melting,
the molecule H_{2}SO_{4} absorbs 960 heat units, and the molecule
H_{2}SO_{4}H_{2}O 3,680 heat units. If therefore we mix one gram
molecule of this hydrate with seventeen gram molecules of snow,
there is an absorption of 18,080 heat units, because 17H_{2}O
absorbs 17 × 1,430 heat units, and the combination of the
monohydrate with water evolves 9,800 heat units. As the specific
heat of the resultant compound H_{2}SO_{4},18H_{2}O = 0·813, the
fall of temperature will be -52°·6. And, in fact, a very low
temperature may be obtained by means of sulphuric acid.

But only a few properties have been determined with sufficient accuracy. In those properties which have been determined for many solutions of sulphuric acid, it is actually seen that the above-mentioned definite compounds are distinguished by distinctive marks of change. As an example we may cite the variation of the specific gravity with a variation of temperature (namely K = _ds/dt_, if _s_ be the sp. gr. and _t_ the temperature). For the normal hydrate, H_{2}SO_{4}, this factor is easily determined from the fact that--

_s_ = 18528 - 10·65_t_ + 0·013_t_^2,

where _s_ is the specific gravity at _t_ (degrees Celsius) if the sp. gr. of water at 4° = 10,000. Therefore K = 10·65 - 0·026_t_. This means that at 0° the sp. gr. of the acid H_{2}SO_{4} decreases by 10·65 for every rise of a degree of temperature, at 10° by 10·39, at 20° by 10·13, at 30° by 9·87.[52] And for solutions containing slightly more anhydride than the acid H_{2}SO_{4} (_i.e._ for fuming sulphuric acid), as well as for solutions containing more water, K is greater than for the acid H_{2}SO_{4}. Thus for the solution SO_{3},2H_{2}SO_{4}, at 10° K = 11·0. On diluting the acid H_{2}SO_{4} K again increases until the formation of the solution H_{2}SO_{4},H_{2}O (K = 11·1 at 10°), and then, on further dilution with water, it again decreases. Consequently both hydrates H_{2}SO_{4} and H_{2}SO_{4},H_{2}O are here expressed by an alteration of the magnitude of K.

[52] For example, if it be taken that at 19° the sp. gr. of pure
sulphuric acid is 1·8330, then at 20° it is 1·8330 - (20 -
19)10·13 = 1·8320.

This shows that in liquid solutions it is possible by studying the variation of their properties (without a change of physical state) to recognise the presence or formation of definite hydrate compounds, and therefore an exact investigation of the properties of solutions, of their specific gravity for instance, should give direct indications of such compounds.[53] The mean result of the most trustworthy determinations of this nature is given in the following tables. The first of these tables gives the specific gravities (in vacuo, taking the sp. gr. of water at 4° = 1), at 0° (column 3), 15° (column 4), and 30° (column 5),[53 bis] for solutions having the composition H_{2}SO_{4} + _n_H_{2}O (the value of _n_ is given in the first column), and containing _p_ (column 2) per cent. (by weight in vacuo) of H_{2}SO_{4}.[53 tri]

_n_ _p_ 0° 15° 30°
100 5·16 1·0374 1·0341 1·0292
50 9·82 1·0717 1·0666 1·0603
25 17·88 1·1337 1·1257 1·1173
15 26·63 1·2040 1·1939 1·1837
10 35·25 1·2758 1·2649 1·2540
8 40·50 1·3223 1·3110 1·2998
6 47·57 1·3865 1·3748 1·3622
5 52·13 1·4301 1·4180 1·4062
4 57·65 1·4881 1·4755 1·4631
3 64·47 1·5635 1·5501 1·5370
2 73·13 1·6648 1·6500 1·6359
1 84·48 1·7940 1·7772 1·7608
0·5 91·59 1·8445 1·8284 1·8128
H_{2}SO_{4} 100 1·8529 1·8372 1·8221

[53] Unfortunately, notwithstanding the great number of fragmentary and
systematic researches which have been made (by Parks, Ure, Bineau,
Kolbe, Lunge, Marignac, Kremers, Thomsen, Perkin, and others) for
determining the relation between the sp. gr. and composition of
solutions of sulphuric acid, they contain discrepancies which
amount to, and even exceed, 0·002 in the sp. gr. For instance, at
15°·4 the solution of composition H_{2}SO_{4}3H_{2}O has a sp. gr.
1·5493 according to Perkin (1886), 1·5501 according to Pickering
(1890), and 1·5525 according to Lunge (1890). The cause of these
discrepancies must be looked for in the methods employed for
determining the composition of the solutions--_i.e._ in the
inaccuracy with which the percentage amount of H_{2}SO_{4} is
determined, for a difference of 1 p.c. corresponds to a difference
of from 0·0070 (for very weak solutions) to 0·0118 (for a solution
containing about 73 p.c.) in the specific gravity (that is the
factor _ds/dp_) at 15°. As it is possible to determine the
specific gravity with an accuracy even exceeding 0·0002, the
specific gravities given in the adjoining tables are only averages
and most probable data in which the error, especially for the
30-80 p.c. solutions cannot be less than 0·0010 (taking water at
4° as 1).

[53 bis] Judging from the best existing determinations (of Marignac,
Kremers, and Pickering) for solutions of sulphuric acid
(especially those containing more than 5 p.c. H_{2}SO_{4}) within
the limits of 0° and 30° (and even to 40°), the variation of the
sp. gr. with the temperature _t_ may (within the accuracy of the
existing determinations) be perfectly expressed by the equation
S_{_t_} = S_{_0_} + A_t_ + B_t_^2. It must be added that (1) three
specific gravities fully determine the variation of the density
with _t_; (2) _ds/dt_ = A + 2B_t_--_i.e._ the factor of the
temperature is expressed by a straight line; (3) the value of A
(if _p_ be greater than 5 p.c.) is negative, and numerically much
greater than B; (4) the value of B for dilute solutions containing
less than 25 p.c. is negative; for solutions approximating to
H_{2}SO_{4}3H_{2}O in their composition it is equal to 0, and for
solutions of greater concentration B is positive; (5) the factor
_ds/dp_ for all temperatures attains a maximum value about
H_{2}SO_{4}H_{2}O; (6) on dividing _ds/dt_ by S_{_0_}, and so
obtaining the coefficient of expansion _k_ (_see_ Note 53), a
minimum is obtained near H_{2}SO_{4} and H_{2}SO_{4}4H_{2}O, and a
maximum at H_{2}SO_{4}H_{2}O for all temperatures.

[53 tri] These data (as well as those in the following table) have been
recalculated by me chiefly upon the basis of Kremer's,
Pickering's, Perkin's, and my own determinations; all the
requisite corrections have been introduced, and I have reason for
thinking that in each of them the probable error (or difference
from the true figures, now unknown) of the specific gravity does
not exceed ±0·0007 (if water at 4° = 1) for the 25-80 p.c.
solutions, and ±0·0002 for the more dilute or concentrated
solutions.

In the second table the first column gives the percentage amount _p_ (by weight) of H_{2}SO_{4}, the second column the weight in grams (S_{15}) of a litre of the solution at 15° (at 4° the weight of a litre of water = 1,000 grams), the third column, the variation (_d_S/_dt_) of this weight for a rise of 1°, the fourth column, the variation _d_S/_dp_ of this weight (at 15°) for a rise of 1 per cent. of H_{2}SO_{4}, the fifth column, the difference between the weight of a litre at 0° and 15° (S_{0} - S_{15}), and the sixth column, the difference between the weight of a litre at 15° and 30° (S_{15} - S_{30}).

_p_ _S__{15} _dS__{15}/_dt_ _dS__{15}/_dp_ _S__{0}- _S__{15}-
_S__{15} _S__{30}
0 999·15 0·148 7·0 0·7 3·4
5 1033·0 0·27 6·8 3·1 5·0
10 1067·7 0·38 7·1 5·2 6·4
20 1141·9 0·58 7·7 8·6 8·9
30 1221·3 0·69 8·2 10·4 10·4
40 1306·6 0·75 8·8 11·3 11·2
50 1397·9 0·79 9·9 11·9 11·8
60 1501·2 0·86 10·8 13·0 12·7
70 1613·1 0·93 11·6 14·1 13·8
80 1731·4 1·04 11·0 15·8 15·4
90 1819·9 1·08 5·4 16·4 16·0
95 1837·6 1·03 +1·7 15·8 15·1
100 1837·2 1·03 -1·9[54] 15·7 15·1

The figures in these tables give the means of finding the amount of H_{2}SO_{4} contained in a solution from its specific gravity,[55] and also show that 'special points' in the lines of variation of the specific gravity with the temperature and percentage composition correspond to certain definite compounds of H_{2}SO_{4} with OH_{2}. This is best seen in the variation of the factors (_d_S/_dt_ and _d_S/_dp_) with the temperature and composition (columns 3, 4, second table). We have already mentioned how the factor of temperature points to the existence of hydrates, H_{2}SO_{4} and H_{2}SO_{4},H_{2}O. As regards the factor _d_S/_dp_ (giving the increase of sp. gr. with an increase of 1 per cent. H_{2}SO_{4}) the following are the three most salient points: (1) In passing from 98 per cent. to 100 per cent. the factor is negative, and at 100 per cent. about -0·0019 (_i.e._ at 99 per cent. the sp. gr. is about 1·8391, and at 100 per cent. about 1·8372, at 15°, the amount of H_{2}SO_{4} has increased whilst the sp. gr. has decreased), but as soon as a certain amount of SO_{3} is added to the definite compound H_{2}SO_{4} (and 'fuming' acid formed) the specific gravity rises (for example, for H_{2}SO_{4} 0·136 SO_{3} the sp. gr. at 15° = 1·866), that is the factor becomes positive (and, in fact, greater by +0·01), so that the formation of the definite hydrate H_{2}SO_{4} is accompanied by a distinct and considerable break in the continuity of the factor[55 bis]; (2) The factor (_d_S/_dp_) in increasing in its passage from dilute to concentrated solutions, attains a maximum value (at 15° about 0·012) about H_{2}SO_{4}2H_{2}O, _i.e._ at about the hydrate corresponding to the form SX_{6}; proper to the compounds of sulphur, for S(OH)_{6} = H_{2}SO_{4}2H_{2}O; the same hydrate corresponds to the composition of gypsum CaSO_{4}2H_{2}O, and to it also corresponds the greatest contraction and rise of temperature in mixing H_{2}SO_{4} with H_{2}O (_see_ Chapter I., Note 28); (3) The variation of the factor (_d_S/_dp_) under certain variations in the composition proceeds so uniformly and regularly, and is so different from the variation given under other proportions of H_{2}SO_{4} and H_{2}O, that the sum of the variations of _d_S/_dp_ is expressed by a series of straight lines, if the values of _p_ be laid along the axis of abscissæ and those of _d_S/_dp_ along the ordinates.[56] Thus, for instance, for 15°, at 10 per cent. _d_S/_dp_ = 0·0071, at 20 per cent. = 0·0077, at 30 per cent. = 0·0082, at 40 per cent. = 0·0088, that is, for each 10 per cent. the factor increases by about 0·0006 for the whole of the above range, but beyond this it becomes larger, and then, after passing H_{2}SO_{4}2H_{2}O, it begins to fall rapidly. Such changes in the variation of the factor take place apparently about definite hydrates,[56 bis] and especially about H_{2}SO_{4}4H_{2}O, H_{2}SO_{4}2H_{2}O and H_{2}SO_{4}H_{2}O. All this indicating as it does the special chemical affinity of sulphuric acid for water, although of no small significance for comprehending the nature of solutions (_see_ Chapter I. and Chapter VII.), contains many special points which require detailed investigation, the chief difficulty being that it requires great accuracy in a large number of experimental data.

[54] The factor _d_S/_dp_ passes through 0, that is, the specific
gravity attains a maximum value at about 98 p.c. This was
discovered by Kohlrausch, and confirmed by Chertel, Pickering, and
others.

[55] Naturally under the condition that there is no other ingredient
besides water, which is sufficiently true. For commercial acid,
whose specific gravity is usually expressed in degrees of Baumé's
hydrometer, we may add that at 15°

Specific gravity 1 1·1 1·2 1·3 1·4 1·5 1·6 1·7 1·8
Degree Baumé 0 13 24 33·3 41·2 48·1 54·1 59·5 64·2

66° Baumé (the strongest commercial acid or oil of vitriol)
corresponds to a sp. gr. 1·84.

By employing the second table (by the method of interpolation) the
specific gravity, at a given temperature (from 0° to 30°) can be
found for any percentage amount of H_{2}SO_{4}, and therefore
conversely the percentage of H_{2}SO_{4} can be found from the
specific gravity.

[55 bis] Whether similar (even small) breaks in the continuity of the
factor _dS_/_dp_ exist or not, for other hydrates (for instance,
for H_{2}SO_{4}H_{2}O and H_{2}SO_{4}4H_{2}O) cannot as yet be
affirmed owing to the want of accurate data (Note 53). In my
investigation of this subject (1887) I admit their possibility,
but only conditionally; and now, without insisting upon a similar
opinion, I only hold to the existence of a distinct break in the
factor at H_{2}SO_{4}, being guided by C. Winkler's observations
ond the specific gravities of fuming sulphuric acid.

[56] In 1887, on considering all the existent observations for a
temperature 0°, I gave the accompanying scheme (p. 243) of the
variation of the factor _ds_/_dp_ at 0°.

I did not then (1887) give this scheme an absolute value, and now
after the appearance of two series of new determinations (Lunge
and Pickering in 1890), which disagree in many points, I think it
well to state quite clearly: (1) that Lunge's and Pickering's new
determinations have not added to the accuracy of our data
respecting the variation of the specific gravity of solutions of
sulphuric acid; (2) that the sum total of existing data does not
negative (within the limit of experimental accuracy) the
possibility of a rectilinear and broken form for the factors
_ds_/_dp_; (3) that the supposition of 'special points' in
_ds_/_dp_, indicating definite hydrates, finds confirmation in all
the latest determinations; (4) that the supposition respecting the
existence of hydrates determining a break of the factor _ds_/_dp_
is in in way altered if, instead of a series of broken straight
lines, there be a continuous series of curves, nearly approaching
straight lines; and (5) that this subject deserves (as I mentioned
in 1887) new and careful elaboration, because it concerns that
foremost problem in our science--solutions--and introduces a
special method into it--that is, the study of differential
variations in a property which is so easily observed as the
specific gravity of a liquid.

[56 bis] These hydrates are: (_a_) H_{2}SO_{4} = SO_{3}H_{2}O (melts
at + 10°·4); (_b_) H_{2}SO_{4}H_{2}O = SO_{3}2H_{2}O
(crystallo-hydrate, melts at +8°·5); (_c_) H_{2}SO_{4}2H_{2}O (is
apparently not crystallisable); (_d_) one of the hydrates between
H_{2}SO_{4}6H_{2}O and H_{2}SO_{4}3H_{2}O, most probably
H_{2}SO_{4}4H_{2}O = SO_{3}5H_{2}O, for it crystallises at -24°·5
(Note 50 bis); and (_e_) a certain hydrate with a large proportion
of water, about H_{2}SO_{4}150H_{2}O. The existence of the last is
inferred from the fact that the factor _ds_/_dp_ first falls,
starting from water, and then rises, and this change takes place
when _p_ is less than 5 p.c. Certainly a change in the variation
of _ds_/_dp_ or _ds_/_dt_ does take place in the neighbourhood of
these five hydrates (Pickering, 1890, recognised a far greater
number of hydrates). I think it well to add that if the
composition of the solutions be expressed by the percentage amount
of molecules--_r__{1}SO_{3} + (100 - _r__{1})H_{2}O we find that
for H_{2}SO_{4}, _r__{1} = 50, for H_{2}SO_{4}2H_{2}O _r__{1} = 25
= 50/2, for H_{2}SO_{4}H_{2}O, _r__{1} = 33·333 = 50 · 2/3, while
for H_{2}SO_{4}4H_{2}O, _r__{1} = 16·666 = 50 · 1/3--_i.e._ that
the chief hydrates are distributed symmetrically between H_{2}O
and H_{2}SO_{4}. Besides which I may mention that my researches
(1887) upon the abrupt changes in the factor for solutions of
sulphuric acid, and upon the correspondence of the breaks of
_ds_/_dp_ with definite hydrates, received an indirect
confirmation not only in the solutions of HNO_{3}, HCl,
C_{2}H_{6}O, C_{3}H_{8}O, &c., which I investigated (in my work
cited in Chapter I., Note 19), but also in the careful
observations made by Professor Cheltzoff on the solutions of
FeCl_{3} and ZnCl_{2} (Chapter XVI., Note 4) which showed the
existence in these solutions of an almost similar change in
_ds_/_dp_ as is found in sulphuric acid. The detailed researches
(1893) made by Tourbaba on the solutions of many organic
substances are of a similar nature. Besides which, H. Crompton
(1888), in his researches on the electrical conductivity of
solutions of sulphuric acid, and Tammann, in his observations on
their vapour tension, found a correlation with the hydrates
indicated as above by the investigation of their specific
gravities. The influence of mixtures of a definite composition
upon the chemical relations of solutions is even exhibited in such
a complex process as electrolysis. V. Kouriloff (1891) showed that
mixtures containing about 3 p.c., 47 p.c. and 73 p.c. of sulphuric
acid--_i.e._ whose composition approaches that of the hydrates
H_{2}SO_{4}150H_{2}O, H_{2}SO_{4}6H_{2}O and
H_{2}SO_{4}2H_{2}O--exhibit certain peculiarities in respect to
the amount of peroxide of hydrogen formed during electrolysis.
Thus a 3 p.c. solution gives a maximum amount of peroxide of
hydrogen at the negative pole, as compared with that given by
other neighbouring concentrations. Starting from 3 p.c., the
formation of peroxide of hydrogen ceases until a concentration of
47 p.c. is reached.

The great affinity of sulphuric acid for water is also seen from the fact that when the strong acid acts on the majority of _organic substances_ containing hydrogen and oxygen (especially on heating) it very frequently _takes up these elements in the form of water_. Thus strong sulphuric acid acting on alcohol, C_{2}H_{6}O, removes the elements of water from it, and converts it into olefiant gas, C_{2}H_{4}. It acts in a similar manner on wood and other vegetable tissues, which it chars. If a piece of wood be immersed in strong sulphuric acid it turns black. This is owing to the fact that the wood contains carbohydrates which give up hydrogen and oxygen as water to the sulphuric acid, leaving charcoal, or a black mass very rich in it. For example, cellulose, C_{6}H_{10}O_{5}, acts in this manner.[57]