[44] Inasmuch as the degree of change exhibited in many properties on

the formation of solutions is not large, so, owing to the

insufficient accuracy of observations, a proportionality between

this change and a change of composition may, in a first rough

approximation and especially within narrow limits of change

of composition, easily be imagined in cases where it does not

even exist. The conclusion of Michel and Kraft is particularly

instructive in this respect; in 1854, on the basis of their

incomplete researches, they supposed that the increment of the

specific gravity of solutions was proportional to the increment

of a salt in a given volume of a solution, which is only true

for determinations of specific gravity which are exact to

the second decimal place--an accuracy insufficient even for

technical determinations. Accurate measurements do not confirm

a proportionality either in this case or in many others where

a ratio has been generally accepted; as, for example, for the

rotatory power (with respect to the plane of polarisation)

of solutions, and for their capillarity, &c. Nevertheless,

such a method is not only still made use of, but even has its

advantages when applied to solutions within a limited scope--as,

for instance, very weak solutions, and for a first acquaintance

with the phenomena accompanying solution, and also as a means

for facilitating the application of mathematical analysis to

the investigation of the phenomenon of solution. Judging by the

results obtained in my researches on the specific gravity of

solutions, I think that in many cases it would be nearer the

truth to take the change of properties as proportional, not to

the amount of a substance dissolved, but to the product of this

quantity and the amount of water in which it is dissolved; the

more so since many chemical relations vary in proportion to the

reacting masses, and a similar ratio has been established for

many phenomena of attraction studied by mechanics. This product

is easily arrived at when the quantity of water in the solutions

to be compared is constant, as is shown in investigating the fall

of temperature in the formation of ice (_see_ footnote 49, p. 91).

[45] All the different forms of chemical reaction may be said to take

place in the process of solution. (1) _Combinations_ between

the solvent and the substance dissolved, which are more or less

stable (more or less dissociated). This form of reaction is the

most probable, and is that most often observed. (2) Reactions

of _substitution_ or of _double decomposition_ between the

molecules. Thus it may be supposed that in the solution of

sal ammoniac, NH_{4}Cl, the action of water produces ammonia,

NH_{4}HO, and hydrochloric acid, HCl, which are dissolved in

the water and simultaneously attract each other. As these

solutions and many others do indeed exhibit signs, which are

sometimes indisputable, of similar double decompositions (thus

solutions of sal-ammoniac yield a certain amount of ammonia),

it is probable that this form of reaction is more often met

with than is generally thought. (3) Reactions of _isomerism_ or

_replacement_ are also probably met with in solution, all the

more as here molecules of different kinds come into intimate

contact, and it is very likely that the configuration of the

atoms in the molecules under these influences is somewhat

different from what it was in its original and isolated state.

One is led to this supposition especially from observations made

on solutions of substances which rotate the plane of polarisation

(and observations of this kind are very sensitive with respect

to the atomic structure of molecules), because they show, for

example (according to Schneider, 1881), that strong solutions

of malic acid rotate the plane of polarisation to the right,

whilst its ammonium salts in all degrees of concentration

rotate the plane of polarisation to the left. (4) Reactions of

_decomposition_ under the influences of solution are not only

rational in themselves, but have in recent years been recognised

by Arrhenius, Ostwald, and others, particularly on the basis of

electrolytic determinations. If a portion of the molecules of a

solution occur in a condition of decomposition, the other portion

may occur in a yet more complex state of combination, just as the

velocity of the motion of different gaseous molecules may be far

from being the same (_see_ Note 34, p. 81).

It is, therefore, very probable that the reactions taking place

in solution vary both quantitatively and qualitatively with

the mass of water in the solution, and the great difficulty

in arriving at a definite conclusion as to the nature of the

chemical relations which take place in the process of solution

will be understood, and if besides this the existence of a

physical process, like the sliding between and interpenetration

of two homogeneous liquids, be also recognised in solution,

then the complexity of the problem as to the actual nature of

solutions, which is now to the fore, appears in its true light.

However, the efforts which are now being applied to the solution

of this problem are so numerous and of such varied aspect that

they will afford future investigators a vast mass of material

towards the construction of a complete theory of solution.

For my part, I am of opinion that the study of the physical

properties of solutions (and especially of weak ones) which now

obtains, cannot give any fundamental and complete solution of

the problem whatever (although it should add much to both the

provinces of physics and chemistry), but that, parallel with it,

should be undertaken the study of the influence of temperature,

and especially of low temperatures, the application to solutions

of the mechanical theory of heat, and the comparative study of

the chemical properties of solutions. The beginning of all this

is already established, but it is impossible to consider in so

short an exposition of chemistry the further efforts of this kind

which have been made up to the present date.

The feeble development of the chemical affinities acting in solutions of

solids becomes evident from those multifarious methods by which _their

solutions are decomposed_, whether they be saturated or not. On heating

(absorption of heat), on cooling, and by internal forces alone, aqueous

solutions in many cases separate into their components or their definite

compounds with water. The water contained in solutions is removed

from them as vapour, or, by freezing, in the form of ice,[46] but the

_tension of the vapour of water_[47] held in solution is less than

that of water in a free state, and the _temperature of the formation

of ice_ from solutions is lower than 0°. Further, both the diminution

of vapour tension and the lowering of the freezing point proceed, in

dilute solutions, almost in proportion to the amount of a substance

dissolved.[48] Thus, if per 100 grams of water there be in solution

1, 5, 10 grams of common salt (NaCl), then at 100° the vapour tension

of the solutions decreases by 4, 21, 43 mm. of the barometric column,

against 760 mm., or the vapour tension of water, whilst the freezing

points are -0·58°, -2·91°, and -6·10° respectively. The above figures[49]

are almost proportional to the amounts of salt in solution (1, 5, and

10 per 100 of water). Furthermore, it has been shown by experiment that

the ratio of the diminution of vapour tension to the vapour tension

of water at different temperatures in a given solution is an almost

constant quantity,[50] and that for every (dilute) solution the ratio

between the diminution of vapour tension and of the freezing point is

also a tolerably constant quantity.[51]

[46] If solutions are regarded as being in a state of dissociation

(_see_ footnote 19, p. 64) it would be expected that they would

contain free molecules of water, which form one of the products

of the decomposition of those definite compounds whose formation

is the cause of solution. In separating as ice or vapour, water

makes, with a solution, a heterogeneous system (made up of

substances in different physical states) similar, for instance,

to the formation of a precipitate or volatile substance in

reactions of double decomposition.

[47] If the substance dissolved is non-volatile (like salt or sugar),

or only slightly volatile, then the whole of the tension of

the vapour given off is due to the water, but if a solution

of a volatile substance--for instance, a gas or a volatile

liquid--evaporates, then only a portion of the pressure belongs

to the water, and the whole pressure observed consists of the sum

of the pressures of the vapours of the water and of the substance

dissolved. The majority of researches bear on the first case,

which will be spoken of presently, and the observations of D. P.

Konovaloff (1881) refer to the second case. He showed that in the

case of two volatile liquids, mutually soluble in each other,

forming two layers of saturated solutions (for example, ether

and water, Note 20, p. 67), both solutions have an equal vapour

tension (in the case in point the tension of both is equal to 431

mm. of mercury at 19·8°). Further, he found that for solutions

which are formed in all proportions, the tension is either

greater (solutions of alcohol and water) or less (solutions of

formic acid) than that which answers to the rectilinear change

(proportional to the composition) from the tension of water

to the tension of the substance dissolved; thus, the tension,

for example, of a 70 p.c. solution of formic acid is less, at

all temperatures, than the tension of water and of formic acid

itself. In this case the tension of a solution is never equal to

the sum of the tensions of the dissolving liquids, as Regnault

already showed when he distinguished this case from that in

which a mixture of liquids, which are insoluble in each other,

evaporates. From this it is evident that a mutual action occurs

in solution, which diminishes the vapour tensions proper to the

individual substances, as would be expected on the supposition of

the formation of compounds in solutions, because the elasticity

then always diminishes.

[48] This amount is usually expressed by the weight of the substance

dissolved per 100 parts by weight of water. Probably it would

be better to express it by the quantity of the substance in a

definite volume of the solution--for instance, in a litre--or

by the ratios of the number of molecules of water and of the

substance dissolved.

[49] The variation of the vapour tension of solutions has been

investigated by many. The best known researches are those of

Wüllner in Germany (1858-1860) and of Tamman in Russia (1887).

The researches on the temperature of the formation of ice from

various solutions are also very numerous; Blagden (1788),

Rüdorff (1861), and De Coppet (1871) established the beginning,

but this kind of investigation takes its chief interest from

the work of Raoult, begun in 1882 on aqueous solutions, and

afterwards continued for solutions in various other easily frozen

liquids--for instance, benzene, C_{6}H_{6} (melts at 4·96°),

acetic acid, C_{2}H_{4}O_{2} (16·75°), and others. An especially

important interest is attached to these cryoscopic investigations

of Raoult in France on the depression of the freezing point,

because he took solutions of many well-known carbon-compounds

and discovered a simple relation between the molecular weight

of the substances and the temperature of crystallisation of the

solvent, which enabled this kind of research to be applied to

the investigation of the nature of substances. We shall meet

with the application of this method later on (_see also_ Chapter

VII.), and at present will only cite the deduction arrived at

from these results. The solution of one-hundredth part of that

molecular gram weight which corresponds with the formula of a

substance dissolved (for example, NaCl = 58·5, C_{2}H_{6}O = 46,

&c.) in 100 parts of a solvent lowers the freezing point of its

solution in water 0·185°, in benzene 0·49°, and in acetic acid

O·39°, or twice as much as with water. And as in weak solutions

the depression or fall of freezing point is proportional to the

amount of the substance dissolved, it follows that the fall of

freezing point for all other solutions may be calculated from

this rule. So, for instance, the weight which corresponds with

the formula of acetone, C_{3}H_{6}O is 58; a solution containing

2·42, 6·22, and 12·35 grams of acetone per 100 grams of water,

forms ice (according to the determinations of Beckmann) at

0·770°, 1·930°, and 3·820°, and these figures show that with

a solution containing 0·58 gram of acetone per 100 of water

the fall of the temperature of the formation of ice will be

0·185°, 0·180°, and 0·179°. It must be remarked that the law of

proportionality between the fall of temperature of the formation

of ice, and the composition of a solution, is in general only

approximate, and is only applicable to weak solutions (Pickering

and others).

We will here remark that the theoretical interest of this subject

was strengthened on the discovery of the connection existing

between the fall of tension, the fall of the temperature of the

formation of ice, of osmotic pressure (Van't Hoff, Note 19),

and of the electrical conductivity of solutions, and we will

therefore supplement what we have already said on the subject by

some short remarks on the method of cryoscopic investigations,

although the details of the subject form the subject of more

special works on physical chemistry (such as Ostwald's _Lehrbuch

der allgemeinen Chemie_, 1891-1894, 2 vols.)

In order to determine the _temperature of the formation of

ice_ (or of crystallisation of other solvents), a solution

of known strength is prepared and poured into a cylindrical

vessel surrounded by a second similar vessel, leaving a layer

of air between the two, which, being a bad conductor, prevents

any rapid change of temperature. The bulb of a sensitive and

corrected thermometer is immersed in the solution, and also a

bent platinum wire for stirring the solution; the whole is then

cooled (by immersing the apparatus in a freezing mixture), and

the temperature at which ice begins to separate observed. If

the temperature at first falls slightly lower, it nevertheless

becomes constant when ice begins to form. By then allowing the

liquid to get just warm, and again observing the temperature of

the formation of ice, an exact determination may be arrived at.

It is still better to take a large mass of solution, and induce

the formation of the first crystals by dropping a small lump

of ice into the solution already partially over-cooled. This

only imperceptibly changes the composition of the solution. The

observation should be made at the point of formation of only a

very small amount of crystals, as otherwise the composition of

the solution will become altered from their separation. Every

precaution must be taken to prevent the access of moisture to the

interior of the apparatus, which might also alter the composition

of the solution or properties of the solvent (for instance, when

using acetic acid).

With respect to the depression of dilute solutions it is

known--(1) That the depression increases in almost direct

proportion to the amount of the substance in solution (always

per 100 parts of water), for example, for KCl when the solution

contains 1 part of salt (per 100 parts of water) the depression =

0·45°, when the solution contains 2 parts of salt = 0·90°, with

10 parts of salt = 4·4°. (2) The greater the molecular weight

expressed by the formula (see Chapter VII.), and designated

by M, the less, under other similar conditions, will be the

depression _d_, and therefore if the concentration of a solution

(the amount by weight of substance dissolved per 100 parts of

water) be designated by _p_, then the fraction M_d_/_p_ or the

molecular depression for a given class of substances will be a

constant quantity; for example, in the case of methyl alcohol in

water 17·3, for acetone about 18·0, for sugar about 18·5. (3) In

general the molecular depression for substances whose solutions

do not conduct an electric current is about 18·5, while for

acids, salts, and such like substances whose solutions do conduct

electricity, it is _i_ times greater; for instance, for HCl, KI,

HNO_{3}, KHO, &c., about 36 (_i_ is nearly 2), for borax about

66, and so on where _i_ varies in the same manner as it does in

the case of the osmotic pressure of solutions (Note 19). (4)

Different solvents (water, acetic acid, benzene, &c.) have each

their corresponding constants of molecular depression (which have

a certain remote connection with their molecular weight); for

example, for acetic acid the molecular depression is about 39 and

not 19 (as it is for water), for benzene 49, for methyl alcohol

about 17, &c. (5) If the molecular weight M of a substance be

unknown, then in the case of non-conductors of electricity or for

a given group, it may be found by determining the depression,

_d_, for a given concentration, _p_; for example, in the case of

peroxide of hydrogen, which is a non-conductor of electricity,

the molecular weight, M, was found to be nearly 34, _i.e._ equal

to H_{2}O_{2}.

Similar results have also been found for the fall in the

vapour tension of solutions (Note 51), and for the rise of

their boiling points (hence these data may also serve for

determining the molecular weight of a substance in solution,

as is shortly described in Chapter VII., Note 27 bis). And as

these conclusions are also applicable in the case of osmotic

pressure (Note 19), and a variation in the magnitude of _i_, in

passing from solutions which do not conduct an electric current

to those which do conduct electricity is everywhere remarked,

so it was natural to here seek that causal connection which

Arrhenius (1888), Ostwald, and others expected to find in the

supposition that a portion of the substance of the electrolyte

is already decomposed in the very act of solution, into its

ions (for example, NaCl into Na and Cl), or into the atoms of

those individual substances which make their appearance in

electrolysis, and in this way to explain the fact that _i_ is

greater for those bodies which conduct an electric current.

We will not consider here this supposition, known as the

hypothesis of 'electrolytic dissociation,' not only because

it wholly belongs to that special branch--physical chemistry,

and gives scarcely any help towards explaining the chemical

relations of solutions (particularly their passage into definite

compounds, their reactions, and their very formation), but

also because--(1) all the above data (for constant depression,

osmotic pressure, &c.) only refer to dilute solutions, and are

not applicable to strong solutions; whilst the chemical interest

in strong solutions is not less than in dilute solutions, and

the transition from the former into the latter is consecutive

and inevitable; (2) because in all homogeneous bodies (although

it may be insoluble and not an electrolyte) a portion of the

atoms may he supposed (Clausius) to be passing from one particle

to another (Chapter X., Note 28), and as it were dissociated,

but there are no reasons for believing that such a phenomenon

is proper to the solutions of electrolytes only; (3) because no

essential mark of difference is observed between the solution of

electrolytes and non-conductors, although it might be expected

there would be according to Arrhenius' hypothesis; (4) because it

is most reasonable to suppose the formation of new, more complex,

but unstable and easily dissociated compounds in the act of

solution, than a decomposition, even partial, of the substances

taken; (5) because if Arrhenius' hypothesis be accepted it

becomes necessary to admit the existence in solutions of free

ions, like the atoms Cl or Na, without any apparent expenditure

of the energy necessary for their disruption, and if in this case

it can be explained why _i_ then = 2, it is not at all clear why

solutions of MgSO_{4} give _i_ = 1, although the solution does

conduct an electric current; (6) because in dilute solutions,

the approximative proportionality between the depression and

concentration may be recognised, while admitting the formation

of hydrates, with as much right as in admitting the solution

of anhydrous substances, and if the formation of hydrates be

recognised it is easier to admit that a portion of these hydrates

is decomposed than to accept the breaking-up into ions; (7)

because the best conductors of electricity are solutions like the

sulphates in which it is necessary to recognise the formation

of associated systems or hydrates; (8) because the cause of

electro-conductivity can be sooner looked for in this affinity

and this combination of the substance dissolved with the solvent,

as is seen from the fact, that (D. P. Konovaloff) neither aniline

nor acetic acid alone conduct an electric current, a solution

of aniline in water conducts it badly (and here the affinity is

very small), while a solution of aniline in acetic acid forms a

good electrolyte, in which, without doubt, chemical forces are

acting, bringing aniline, like ammonia, into combination with the

acetic acid; which is evident from the researches made by Prof.

Konovaloff upon mixtures (solutions) of aniline and other amines;

and, lastly, (9) because I, together with many of the chemists

of the present day, cannot regard the hypothesis of electrolytic

dissociation in the form given to it up to now by Arrhenius and

Ostwald, as answering to the sum total of the chemical data

respecting solutions and dissociation in general. Thus, although

I consider it superfluous to discuss further the evolution of

the above theory of solutions, still I think that it would he

most useful for students of chemistry to consider all the data

referring to this subject, which can be found in the _Zeitschrift

für physikalische Chemie_, 1888-1894.

[50] This fact, which was established by Gay-Lussac, Pierson, and

v. Babo, is confirmed by the latest observations, and enables us

to express not only the fall of tension (_p_-_p_´) itself, but

its ratio to the tension of water (_p_-_p_´)/_p_. It is to be

remarked that in the absence of any chemical action, the fall of

pressure is either very small, or does not exist at all (note

33), and is not proportional to the quantity of the substance

added. As a rule, the tension is then equal, according to the law

of Dalton, to the sum of the tensions of the substances taken.

Hence liquids which are insoluble in each other (for example,

water and chloride of carbon) present a tension equal to the sum

of their individual tensions, and therefore such a mixture boils

at a lower temperature than the more volatile liquid (Magnus,

Regnault).

[51] If, in the example of common salt, the fall of tension be divided

by the tension of water, a figure is obtained which is nearly

105 times less than the magnitude of the fall of temperature of

formation of ice. This correlation was theoretically deduced

by Goldberg, on the basis of the application of the mechanical

theory of heat, and is repeated by many investigated solutions.

The diminution of the vapour tension of solutions explains the rise in

boiling point due to the solution of solid non-volatile bodies in water.

The temperature of a vapour is the same as that of the solution from

which it is generated, and therefore it follows that the aqueous vapour

given off from a solution will be superheated. A saturated solution

of common salt boils at 108·4°, a solution of 335 parts of nitre in

100 parts of water at 115·9°, and a solution of 325 parts of potassium

chloride in 100 parts of water at 179°, if the temperature of ebullition

be determined by immersing the thermometer bulb in the liquid itself.

This is another proof of the bond which exists between water and the

substance dissolved. And this bond is seen still more clearly in those

cases (for example, in the solution of nitric or formic acid in water)

where the solution boils at a higher temperature than either water or

the volatile substance dissolved in it. For this reason the solutions of

certain gases--for instance, hydriodic or hydrochloric acid--boil above

100°.

The separation of ice from solutions[52] explains both the phenomenon,

well known to sailors, that the ice formed from salt water gives fresh

water, and also the fact that by freezing, just as by evaporation, a

solution is obtained which is richer in salts than before. This is taken

advantage of in cold countries for obtaining a liquor from sea water,

which is then evaporated for the extraction of salt.

[52] Fritzsche showed that solutions of certain colouring matters yield

colourless ice, which clearly proves the passage of water only

into a solid state, without any intermixture of the substance

dissolved, although the possibility of the admixture in certain

other cases cannot be denied.

On the removal of part of the water from a solution (by evaporation

or the separation of ice), a saturated solution should be obtained,

and then the solid substance dissolved should separate out. Solutions

saturated at a certain temperature should also separate out a

corresponding portion of the substance dissolved if they be reduced,

by cooling,[53] to a temperature at which the water can no longer hold

the former quantity of the substance in solution. If this separation,

by cooling a saturated solution or by evaporation, take place slowly,

_crystals_ of the substance dissolved are in many cases formed; and

this is the method by which crystals of soluble salts are usually

obtained. Certain solids very easily separate out from their solutions

in perfectly formed crystals, which may attain very large dimensions.

Such are nickel sulphate, alum, sodium carbonate, chrome-alum, copper

sulphate, potassium ferricyanide, and a whole series of other salts.

The most remarkable circumstance in this is that many solids in

separating out from an aqueous solution retain a portion of water,

forming crystallised solid substances which contain water. A portion

of the water previously in the solution remains in the separated

crystals. The water which is thus retained is called the _water of

crystallisation_. Alum, copper sulphate, Glauber's salt, and magnesium

sulphate contain such water, but neither sal-ammoniac, table salt,

nitre, potassium chlorate, silver nitrate, nor sugar, contains any

water of crystallisation. One and the same substance may separate out

from a solution with or without water of crystallisation, according

to the temperature at which the crystals are formed. Thus common salt

in crystallising from its solution in water at the ordinary or at a

higher temperature does not contain water of crystallisation. But if

its separation from the solution takes place at a low temperature,

namely below -5°, then the crystals contain 38 parts of water in 100

parts. Crystals of the same substance which separate out at different

temperatures may contain different amounts of water of crystallisation.

This proves to us that a solid dissolved in water may form various

compounds with it, differing in their properties and composition,

and capable of appearing in a solid separate form like many ordinary

definite compounds. This is indicated by the numerous properties and

phenomena connected with solutions, and gives reason for thinking that

there exist in solutions themselves such compounds of the substance

dissolved, and the solvent or compounds similar to them, only in a

liquid partly decomposed form. Even the _colour of solutions_ may

often confirm this opinion. Copper sulphate forms crystals having a

blue colour and containing water of crystallisation. If the water

of crystallisation be removed by heating the crystals to redness, a

colourless anhydrous substance is obtained (a white powder). From this

it may be seen that the blue colour belongs to the compound of the

copper salt with water. Solutions of copper sulphate are all blue, and

consequently they contain a compound similar to the compound formed by

the salt with its water of crystallisation. Crystals of cobalt chloride

when dissolved in an anhydrous liquid--like alcohol, for instance--give

a blue solution, but when they are dissolved in water a red solution is

obtained. Crystals from the aqueous solution, according to Professor

Potilitzin, contain six times as much water (CoCl_{2},6H_{2}O) for a

given weight of the salt, as those violet crystals (CoCl_{2},H_{2}O)

which are formed by the evaporation of an alcoholic solution.

[53] As the solubility of certain substances (for example, coniine,

cerium sulphate, and others) decreases with a rise of temperature

(between certain limits--see, for example, note 24), so these

substances do not separate from their saturated solutions on

cooling but on heating. Thus a solution of manganese sulphate,

saturated at 70°, becomes cloudy on further heating. The point at

which a substance separates from its solution with a change of

temperature gives an easy means of determining the co-efficient

of solubility, and this was taken advantage of by Prof. Alexéeff

for determining the solubility of many substances. The phenomenon

and method of observation are here essentially the same as in

the determination of the temperature of formation of ice. If a

solution of a substance which separates out on heating be taken

(for example, the sulphate of calcium or manganese), then at a

certain fall of temperature ice will separate out from it, and

at a certain rise of temperature the salt will separate out.

From this example, and from general considerations, it is clear

that the separation of a substance dissolved from a solution

should present a certain analogy to the separation of ice from a

solution. In both cases, a heterogeneous system of a solid and a

liquid is formed from a homogeneous (liquid) system.

That solutions contain particular compounds with water is further shown

by the phenomena of supersaturated solutions, of so-called cryohydrates,

of solutions of certain acids having constant boiling points, and the

properties of compounds containing water of crystallisation whose data

it is indispensable to keep in view in the consideration of solutions.

Supersaturated solutions exhibit the following phenomena:--On the

refrigeration of a saturated solution of certain salts,[54] if the

liquid be brought under certain conditions, the excess of the solid

may sometimes remain in solution and not separate out. A great number

of substances, and more especially sodium sulphate, Na_{2}SO_{4}, or

Glauber's salt, easily form supersaturated solutions. If boiling water

be saturated with this salt, and the solution be poured off from any

remaining undissolved salt, and, the boiling being still continued,

the vessel holding the solution be well closed by cotton wool, or by

fusing up the vessel, or by covering the solution with a layer of oil,

then it will he found that this saturated solution does not separate

out any Glauber's salt whatever on cooling down to the ordinary or even

to a much lower temperature; although without the above precautions a

salt separates out on cooling, in the form of crystals, which contain

Na_{2}SO_{4},10H_{2}O--that is, 180 parts of water for 142 parts of

anhydrous salt. The supersaturated solution may be moved about or

shaken inside the vessel holding it, and no crystallisation will take

place; the salt remains in the solution in as large an amount as at a

higher temperature. If the vessel holding the supersaturated solution

be opened and a crystal of Glauber's salt be thrown in, crystallisation

suddenly takes place.[55] A considerable rise in temperature is noticed

during this rapid separation of crystals, which is due to the fact

that the salt, previously in a liquid state, passes into a solid state.

This bears some resemblance to the fact that water maybe cooled below

0° (even to -10°) if it be left at rest, under certain circumstances,

and evolves heat in suddenly crystallising. Although from this point

of view there is a resemblance, yet in reality the phenomenon of

supersaturated solutions is much more complicated. Thus, on cooling,

a saturated solution of Glauber's salt deposits crystals containing

Na_{2}SO_{4},7H_{2}0,[56] or 126 parts of water per 142 parts of

anhydrous salt, and not 180 parts of water, as in the above-mentioned

salt. The crystals containing 7H_{2}O are distinguished for their

instability; if they stand in contact not only with crystals of

Na_{2}SO_{4},10H_{2}O, but with many other substances, they immediately

become opaque, forming a mixture of anhydrous and deca-hydrated salts.

It is evident that between water and a soluble substance there may be

established different kinds of greater or less stable equilibrium, of

which solutions form a particular case.[57]

[54] Those salts which separate out with water of crystallisation and

give several crystallohydrates form supersaturated solutions with

the greatest facility, and the phenomenon is much more common

than was previously imagined. The first data were given in the

last century by Loewitz, in St. Petersburg. Numerous researches

have proved that supersaturated solutions do not differ from

ordinary solutions in any of their essential properties. The

variations in specific gravity, vapour tension, formation of ice,

&c., take place according to the ordinary laws.

[55] Inasmuch as air, as has been shown by direct experiment, contains,

although in very small quantities, minute crystals of salts,

and among them sodium sulphate, air can bring about the

crystallisation of a supersaturated solution of sodium sulphate

in an open vessel, but it has no effect on saturated solutions of

certain other salts; for example, lead acetate. According to the

observations of De Boisbaudran, Gernez, and others, isomorphous

salts (analogous in composition) are capable of inducing

crystallisation. Thus, a supersaturated solution of nickel

sulphate crystallises by contact with crystals of sulphates of

other metals analogous to it, such as those of magnesium, cobalt,

copper, and manganese. The crystallisation of a supersaturated

solution, set up by the contact of a minute crystal, starts from

it in rays with a definite velocity, and it is evident that the

crystals as they form propagate the crystallisation in definite

directions. This phenomenon recalls the evolution of organisms

from germs. An attraction of similar molecules ensues, and they

dispose themselves in definite similar forms.

[56] At the present time a view is very generally accepted, which

regards supersaturated solutions as homogeneous systems, which

pass into heterogeneous systems (composed of a liquid and a

solid substance), in all respects exactly resembling the passage

of water cooled below its freezing point into ice and water,

or the passage of crystals of rhombic sulphur into monoclinic

crystals, and of the monoclinic crystals into rhombic. Although

many phenomena of supersaturation are thus clearly understood,

yet the spontaneous formation of the unstable hepta-hydrated salt

(with 7H_{2}O), in the place of the more stable deca-hydrated

salt (with mol. 10H_{2}O), indicates a property of a saturated

solution of sodium sulphate which obliges one to admit that

it has a different structure from an ordinary solution.

Stcherbacheff asserts, on the basis of his researches, that

a solution of the deca-hydrated salt gives, on evaporation,

without the aid of heat, the deca-hydrated salt, whilst after

heating above 33° it forms a supersaturated solution and the

hepta-hydrated salt. But in order that this view should be

accepted, some facts must be discovered distinguishing solutions

(which are, according to this view, isomeric) containing the

hepta-hydrated salt from those containing the deca-hydrated

salt, and all efforts in this direction (the study of the

properties of the solutions) have given negative results. As

some crystallohydrates of salts (alums, sugar of lead, calcium

chloride) melt straightway (without separating out anything),

whilst others (like Na_{2}SO_{4},10H_{2}O) are broken up, then

it may be that the latter are only in a state of equilibrium at

a higher temperature than their melting point. It may here be

observed that in melting crystals of the deca-hydrated salt,

there is formed, besides the solid anhydrous salt, a saturated

solution giving the hepta-hydrated salt, so that this passage

from the deca-to the hepta-hydrated salt, and the reverse,

takes place with the formation of the anhydrous (or, it may be,

monohydrated) salt.

Moreover, supersaturation (Potilitzin, 1889) only takes

place with those substances which are capable of giving

several modifications or several crystallohydrates, _i.e._

supersaturated solutions separate out, besides the stable normal

crystallohydrate, hydrates containing less water and also the

anhydrous salt. This degree of saturation acts upon the substance

dissolved in a like manner to heat. Sulphate of nickel in a

solution at 15° to 20° separates out rhombic crystals with

7H_{2}O, at 30° to 40° cubical crystals, with 6H_{2}O, at 50° to

70° monoclinic crystals, also containing 6H_{2}O. Crystals of

the same composition separate out from supersaturated solutions

at one temperature (17° to 19°), but at different degrees of

saturation, as was shown by Lecoq de Boisbaudran. The capacity

to voluntarily separate out slightly hydrated or anhydrous salts

by the introduction of a crystal into the solution is common to

all supersaturated solutions. If a salt forms a supersaturated

solution, then one would expect, according to this view, that

it should exist in the form of several hydrates or in several

modifications. Thus Potilitzin concluded that chlorate of

strontium, which easily gives supersaturated solutions, should

be capable of forming several hydrates, besides the anhydrous

salt known; and he succeeded in discovering the existence

of two hydrates, Sr(ClO_{3})_{2},3H_{2}O and apparently

Sr(ClO_{3})_{2},8H_{2}O. Besides this, three modifications of the

common anhydrous salt were obtained, differing from each other

in their crystalline form. One modification separated out in the

form of rhombic octahedra, another in oblique plates, and a third

in long brittle prisms or plates. Further researches showed that

salts which are not capable of forming supersaturated solutions

such as the bromates of calcium, strontium, and barium, part

with their water of hydration with difficulty (they crystallise

with 1H_{2}O), and decompose very slowly in a vacuum or in dry

air. In other words the tension of dissociation is very small

in this class of hydrates. As the hydrates characterised by a

small dissociation tension are incapable of giving supersaturated

solutions, so conversely supersaturated solutions give hydrates

whose tension of dissociation is great (Potilitzin, 1893).

[57] _Emulsions_, like milk, are composed of a solution of glutinous

or similar substances, or of oily liquids suspended in a

liquid in the form of drops, which are clearly visible under a

microscope, and form an example of a mechanical formation which

resembles solution. But the difference from solutions is here

evident. There are, however, solutions which approach very near

to emulsions in the facility with which the substance dissolved

separates from them. It has long been known, for example, that

a particular kind of Prussian blue, KFe_{2}(CN)_{6}, dissolves

in pure water, but, on the addition of the smallest quantity

of either of a number of salts, it coagulates and becomes

quite insoluble. If copper sulphide (CuS), cadmium sulphide

(CdS), arsenic sulphide (As_{2}S_{5}) (the experiments with

these substances proceed with great ease, and the solution

obtained is comparatively stable), and many other metallic

sulphides, be obtained by a method of double decomposition (by

precipitating salts of these metals by hydrogen sulphide),

and be then carefully washed (by allowing the precipitate to

settle, pouring off the liquid, and again adding sulphuretted

hydrogen water), then, as was shown by Schulze, Spring, Prost,

and others, the previously insoluble sulphides pass into

transparent (for mercury, lead, and silver, reddish brown; for

copper and iron, greenish brown; for cadmium and indium, yellow;

and for zinc, colourless) solutions, which may be preserved

(the weaker they are the longer they keep) and even boiled, but

which, nevertheless, in time coagulate--that is, separate in an

insoluble form, and then sometimes become crystalline and quite

incapable of re-dissolving. Graham and others observed the power

shown by colloids (_see_ note 18) of forming similar _hydrosols

or solutions of gelatinous colloids_, and, in describing alumina

and silica, we shall again have occasion to speak of such

solutions.

In the existing state of our knowledge concerning solution, such

solutions may be looked on as a transition between emulsion and

ordinary solutions, but no fundamental judgment can be formed

about them until a study has been made of their relations to

ordinary solutions (the solutions of even soluble colloids freeze

immediately on cooling below 0°, and, according to Guthrie, do

not form cryohydrates), and to supersaturated solutions, with

which they have certain points in common.

Solutions of salts on refrigeration below 0° deposit ice or crystals

(which then frequently contain water of crystallisation) of the salt

dissolved, and on reaching a certain degree of concentration they

solidify in their entire mass. These solidified masses are termed

_cryohydrates_. My researches on solutions of common salt (1868) showed

that its solution solidifies when it reaches a composition NaCl +

10H_{2}O (180 parts of water per 58·5 parts of salt), which takes place

at about -23°. The solidified solution melts at the same temperature,

and both the portion melted and the remainder preserve the above

composition. Guthrie (1874-1876) obtained the cryohydrates of many

salts, and he showed that certain of them are formed like the above at

comparatively low temperatures, whilst others (for instance, corrosive

sublimate, alums, potassium chlorate, and various colloids) are formed

on a slight cooling, to -2° or even before.[58] In the case of common

salt, the cryohydrate with 10 molecules of water, and in the case of

sodium nitrate, the cryohydrate[59] with 7 molecules of water (_i.e._

126 parts of water per 85 of salt) should be accepted as established

substances, capable of passing from a solid to a liquid state and

conversely; and therefore it may be thought that in cryohydrates we

have solutions which are not only undecomposable by cold, but also have

a definite composition which would present a fresh case of definite

equilibrium between the solvent and the substance dissolved.

[58] Offer (1880) concludes, from his researches on cryohydrates, that

they are simple mixtures of ice and salts, having a constant

melting point, just as there are alloys having a constant point

of fusion, and solutions of liquids with a constant boiling point

(_see_ note 60). This does not, however, explain in what form

a salt is contained, for instance, in the cryohydrate NaCl +

10H_{2}O. At temperatures above -10° common salt separates out in

anhydrous crystals, and at temperatures near -10°, in combination

with water of crystallisation, NaCl + 2H_{2}O, and, therefore,

it is very improbable that at still lower temperatures it would

separate without water. If the possibility of the solidified

cryohydrate containing NaCl + 2H_{2}O and ice be admitted, then

it is not clear why one of these substances does not melt before

the other. If alcohol does not extract water from the solid mass,

leaving the salt behind, this does not prove the presence of ice,

because alcohol also takes up water from the crystals of many

hydrated substances (for instance, from NaCl + 2H_{2}O) at about

their melting-points. Besides which, a simple observation on the

cryohydrate, NaCl + 10H_{2}O, shows that with the most careful

cooling it does not on the addition of ice deposit ice, which

would occur if ice were formed on solidification intermixed with

the salt.

I may add with regard to cryohydrates that many of the solutions

of acids solidify completely on prolonged cooling (for example,

H_{2}SO_{4},H_{2}O), and then form perfectly definite compounds.

For the solutions of sulphuric acid (_see_ Chapter XX.)

Pickering obtained, for instance, a hydrate, H_{2}SO_{4},4H_{2}O

at -25°. Hydrochloric, nitric, and other acids also give similar

crystalline hydrates, melting at low temperatures and presenting

many similarities with the cryohydrates.

[59] _See_ note 24.

The formation of definite but unstable compounds in the process of

solution becomes evident from the phenomena of a marked decrease of

vapour tension, or from the rise of the temperature of ebullition which

occurs in the solution of certain volatile liquids and gases in water.

As an example, we will take hydriodic acid, HI, a gas which liquefies,

giving a liquid which boils at -20°. A solution of it containing 57 p.c.

of hydriodic acid is distinguished by the fact that if it be heated

the hydriodic acid volatilises together with the water in the same

proportions as they occur in the solution, therefore such a solution

may be distilled unchanged. The solution boils at a higher temperature

than water, at 127°. A portion of the physical properties of the gas

and water have in this case already disappeared--a new substance is

formed, which has its definite boiling point. To put it more correctly,

this is not the temperature of ebullition, but the temperature at which

the compound formed decomposes, forming the vapours of the products of

dissociation, which, on cooling, re-combine. Should a less amount of

hydriodic acid be dissolved in water than the above, then, on heating

such a solution, water only at first distils over, until the solution

attains the above-mentioned composition; it will then distil over

unaltered. If more hydriodic acid be passed into such a solution a fresh

quantity of the gas will dissolve, but it passes off with great ease,

like air from water. It must not, however, be thought that those forces

which determine the formation of ordinary gaseous solutions play no part

whatever in the formation of a solution having a definite boiling point;

that they do react is shown from the fact that such constant gaseous

solutions vary in their composition under different pressures.[60] It

is not, therefore, at every, but only at the ordinary, atmospheric

pressure that a constant boiling solution of hydriodic acid will contain

57 p.c. of the gas. At another pressure the proportion of water and

hydriodic acid will be different. It varies, however, judging from

observations made by Roscoe, very little for considerable variations of

pressure. This variation in composition directly indicates that pressure

exerts an influence on the formation of unstable chemical compounds

which are easily dissociated (with formation of a gas), just as it

influences the solution of gases, only the latter is influenced to a

more considerable degree than the former.[61] Hydrochloric, nitric, and

other acids form _solutions having definite boiling points_, like that

of hydriodic acid. They show further the common property, if containing

but a small proportion of water, that they _fume in air_. Strong

solutions of nitric, hydrochloric, hydriodic, and other gases are even

termed 'fuming acids.' The fuming liquids contain a definite compound

whose temperature of ebullition (decomposition) is higher than 100°,

and contain also an excess of the volatile substance dissolved, which

exhibits a capacity to combine with water and form a hydrate, whose

vapour tension is less than that of aqueous vapour. On evaporating in

air, this dissolved substance meets the atmospheric moisture and forms

a visible vapour (fumes) with it, which consists of the above-mentioned

compound. The attraction or affinity which binds, for instance,

hydriodic acid with water is evinced not only in the evolution of heat

and the diminution of vapour tension (rise of boiling point), but also

in many purely chemical relations. Thus hydriodic acid is produced from

iodine and hydrogen sulphide in the presence of water, but unless water

is present this reaction does not take place.[62]

[60] For this reason (the want of entire constancy of the composition

of constant boiling solutions with a change of pressure), the

existence of definite hydrates formed by volatile substances--for

instance, by hydrochloric acid and water--is frequently denied.

It is generally argued as follows: If there did exist a constancy

of composition, then it would be unaltered by a change of

pressure. But the distillation of constant boiling hydrates

is undoubtedly accompanied (judging by the vapour densities

determined by Bineau), like the distillation of sal ammoniac,

sulphuric acid, &c., by a complete decomposition of the original

compound--that is, these substances do not exist in a state

of vapour, but their products of decomposition (hydrochloric

acid and water) are gases at the temperature of volatilisation,

which dissolve in the volatilised and condensed liquids; but

the solubility of gases in liquids depends on the pressure,

and, therefore, the composition of constant boiling solutions

may, and even ought to, vary with a change of pressure, and,

further, the smaller the pressure and the lower the temperature

of volatilisation, the more likely is a true compound to be

obtained. According to the researches of Roscoe and Dittmar

(1859), the constant boiling solution of hydrochloric acid

proved to contain 18 p.c. of hydrochloric acid at a pressure of

3 atmospheres, 20 p.c. at 1 atmosphere, and 23 p.c. at 1/10 of

an atmosphere. On passing air through the solution until its

composition became constant (_i.e._ forcing the excess of aqueous

vapour or of hydrochloric acid to pass away with the air),

then acid was obtained containing about 20 p.c. at 100°, about

23 p.c. at 50°, and about 25 p.c. at 0°. From this it is seen

that by decreasing the pressure and lowering the temperature of

evaporation one arrives at the same limit, where the composition

should be taken as HCl + 6H_{2}O, which requires 25·26 p.c. of

hydrochloric acid. Fuming hydrochloric acid contains more than

this.

In the case already considered, as in the case of formic acid

in the researches of D. P. Konovaloff (note 47), the constant

boiling solution corresponds with a minimum tension--that

is, with a boiling point higher than that of either of the

component elements. But there is another case of constant boiling

solutions similar to the case of the solution of propyl alcohol,

C_{3}H_{8}O, when a solution, undecomposed by distillation, boils

at a lower point than that of the more volatile liquid. However,

in this case also, if there be solution, the possibility of the

formation of a definite compound in the form C_{3}H_{8}O + H_{2}O

cannot be denied, and the tension of the solution is not equal to

the sum of tensions of the components. There are possible cases

of constant boiling mixtures even when there is no solution nor

any loss of tension, and consequently no chemical action, since

the amount of liquids that are volatilised is determined by

the product of the vapour densities into their vapour tensions

(Wanklyn), in consequence of which liquids whose boiling point

is above 100°--for instance, turpentine and ethereal oils in

general--when distilled with aqueous vapour, pass over at a

temperature below 100°. Consequently, it is not in the constancy

of composition and boiling point (temperature of decomposition)

that evidence of a distinct chemical action is to be found in

the above-described solutions of acids, but in the great loss

of tension, which completely resembles the loss of tension

observed, for instance, in the perfectly-definite combinations of

substances with water of crystallisation (see later, note 65).

Sulphuric acid, H_{2}SO_{4}, as we shall learn later, is also

decomposed by distillation, like HCl + 6H_{2}O, and exhibits,

moreover, all the signs of a definite chemical compound. The

study of the variation of the specific gravities of solutions as

dependent on their composition (see note 19) shows that phenomena

of a similar kind, although of different dimensions, take place

in the formation of both H_{2}SO_{4} from H_{2}O and SO_{3}, and

of HCl + 6H_{2}O (or of aqueous solutions analogous to it) from

HCl and H_{2}O.

[61] The essence of the matter may he thus represented. A gaseous or

easily volatile substance _A_ forms with a certain quantity of

water, _n_H_{2}O, a definite complex compound _An_H_{2}O, which

is stable up to a temperature t° higher than 100°. At this

temperature it is decomposed into two substances, _A_ + H_{2}O.

Both boil below _t_° at the ordinary pressure, and therefore at

_t_° they distil over and re-combine in the receiver. But if a

part of the substance _An_H_{2}O is decomposed or volatilised,

a portion of the undecomposed liquid still remains in the

vessel, which can partially dissolve one of the products of

decomposition, and that in quantity varying with the pressure and

temperature, and therefore the solution at a constant boiling

point will have a slightly different composition at different

pressures.

[62] For solutions of hydrochloric acid in water there are still greater

differences in reactions. For instance, strong solutions

decompose antimony sulphide (forming hydrogen sulphide, H_{2}S),

and precipitate common salt from its solutions, whilst weak

solutions do not act thus.

Many compunds containing water of crystallisation are solid substances

(when melted they are already solutions--_i.e._ liquids); furthermore,

they are capable of being formed from solutions, like ice or aqueous

vapour. They may be called _crystallo-hydrates_. Inasmuch as the direct

presence of ice or aqueous vapour cannot be admitted in solutions

(for these are liquids), although the presence of water may be, so

also there is no basis for acknowledging the presence in solutions

of crystallo-hydrates, although they are obtained from solutions as

such.[63] It is evident that such substances present one of the many

forms of equilibrium between water and a substance dissolved in it.

This form, however, reminds one, in all respects, of solutions--that

is, aqueous compounds which are more or less easily decomposed, with

separation of water and the formation of a less aqueous or an anhydrous

compound. In fact, there are not a few crystals containing water which

lose a part of their water at the ordinary temperature. Of such a kind,

for instance, are the crystals of soda, or sodium carbonate, which,

when separated from an aqueous solution at the ordinary temperature,

are quite transparent; but when left exposed to air, lose a portion

of their water, becoming opaque, and, in the process, lose their

crystalline appearance, although preserving their original form.

This process of the separation of water at the ordinary temperature

is termed the _efflorescence_ of crystals. Efflorescence takes place

more rapidly under the receiver of an air pump, and especially at a

gentle heat. This breaking up of a crystal is dissociation at the

ordinary temperature. Solutions are decomposed in exactly the same

manner.[64] The tension of the aqueous vapour which is given off

from crystallo-hydrates is naturally, as with solutions, less than

the vapour tension of water itself[65] at the same temperature, and

therefore many anhydrous salts which are capable of combining with

water absorb aqueous vapour from moist air; that is, they act like a

cold body on which water is deposited from steam. It is on this that

the desiccation of gases is based, and it must further be remarked in

this respect that certain substances--for instance, potassium carbonate

(K_{2}CO_{3}) and calcium chloride (CaCl_{2})--not only absorb the water

necessary for the formation of a solid crystalline compound, but also

give solutions, or _deliquesce_, as it is termed, in moist air. Many

crystals do not effloresce in the least at the ordinary temperature;

for example, copper sulphate, which may be preserved for an indefinite

length of time without efflorescing, but when placed under the receiver

of an air pump, if efflorescence be once started, it goes on at the

ordinary temperature. The temperature at which the complete separation

of water from crystals takes place varies considerably, not only for

different substances, but also for different portions of the contained

water. Very often the temperature at which dissociation begins is very

much higher than the boiling point of water. So, for example, copper

sulphate, which contains 36 p.c. of water, gives up 28·8 p.c. at 100°,

and the remaining quantity, namely 7·2 p.c., only at 240°. Alum, out of

the 45·5 p.c. of water which it contains, gives up 18·9 p.c. at 100°,

17·7 p.c. at 120°, 7·7 p.c. at 180°, and 1 p.c. at 280°; it only loses

the last quantity (1 p.c.) at its temperature of decomposition. These

examples clearly show that the annexation of water of crystallisation

is accompanied by a rather profound, although, in comparison with

instances which we shall consider later, still inconsiderable, change

of its properties. In certain cases the water of crystallisation is

only given off when the solid form of the substance is destroyed: when

the crystals melt on heating. The crystals are then said _to melt in

their water of crystallisation_. Further, after the separation of the

water, a solid substance remains behind, so that by further heating it

acquires a solid form. This is seen most clearly in crystals of sugar of

lead or lead acetate, which melt in their water of crystallisation at a

temperature of 56·25°, and in so doing begin to lose water. On reaching

a temperature of 100° the sugar of lead solidifies, having lost all its

water; and then at a temperature of 280°, the anhydrous and solidified

salt again melts.[65 bis]

[63] Supersaturated solutions give an excellent proof in this respect.

Thus a solution of copper sulphate generally crystallises in

penta-hydrated crystals, CuSO_{4} + 5H_{2}O, and its saturated

solution gives such crystals if it be brought into contact with

the minutest possible crystal of the same kind. But, according

to the observations of Lecoq de Boisbaudran, if a crystal of

ferrous sulphate (an isomorphous salt, _see_ note 55), FeSO_{4}

+ 7H_{2}O, be placed in a saturated solution of copper sulphate,

then crystals of hepta-hydrated salt, CuSO_{4} + 7H_{2}O,

are obtained. It is evident that neither the penta-nor the

hepta-hydrated salt is contained as such in the solution. The

solution presents its own particular liquid form of equilibrium.

[64] Efflorescence, like every evaporation, proceeds from the surface.

In the interior of crystals which have effloresced there is

usually found a non-effloresced mass, so that the majority of

effloresced crystals of washing soda show, in their fracture, a

transparent nucleus coated by an effloresced, opaque, powdery

mass. It is a remarkable circumstance in this respect that

efflorescence proceeds in a completely regular and uniform

manner, so that the angles and planes of similar crystallographic

character effloresce simultaneously, and in this respect the

crystalline form determines those parts of crystals where

efflorescence starts, and the order in which it continues. In

solutions evaporation also proceeds from the surface, and the

first crystals which appear on its reaching the required degree

of saturation are also formed at the surface. After falling to

the bottom the crystals naturally continue to grow (_see_ Chapter

X.).

[65] According to Lesc[oe]ur (1883), at 100° a concentrated solution of

barium hydroxide, BaH_{2}O_{2}, on first depositing crystals

(with + H_{2}O) has a tension of about 630 mm. (instead of

760 mm., the tension of water), which decreases (because the

solution evaporates) to 45 mm., when all the water is expelled

from the crystals, BaH_{2}O_{2} + H_{2}O, which are formed, but

they also lose water (dissociate, effloresce at 100°), leaving

the hydroxide, BaH_{2}O_{2}, which is perfectly undecomposable

at 100°--that is, does not part with water. At 73° (the tension

of water is then 265 mm.) a solution, containing 33H_{2}O,

on crystallising has a tension of 230 mm.; the crystals,

BaH_{2}O_{2} + 8H_{2}O, which separate out, have a tension of

160 mm.; on losing water they give BaH_{2}O_{2} + H_{2}O. This

substance does not decompose at 73°, and therefore its tension =

0. In those crystallohydrates which effloresce at the ordinary

temperature, the tension of dissociation nearly approximates

to that of the aqueous vapour, as Lesc[oe]ur (1891) showed. To

this category of compounds belong B_{2}O_{3}(3 + _x_)H_{2}O,

C_{2}O_{4}H_{2}(2 + _x_)H_{2}O, BaO(9 + _x_)H_{2}O, and SrO(9

+ _x_)H_{2}O. And a still greater tension is possessed by

Na_{2}SO_{4}10H_{2}O, Na_{2}CO_{3}10H_{2}O, and MgSO_{4}(7 +

_x_)H_{2}O. Müller-Erzbach (1884) determines the tension (with

reference to liquid water) by placing tubes of the same length

with water and the substances experimented with in a desiccator,

the rate of loss of water giving the relative tension. Thus,

at the ordinary temperature, crystals of sodium phosphate,

Na_{2}HPO_{4} + 12H_{2}O, present a tension of 0·7 compared

with water, until they lose 5H_{2}O, then 0·4 until they lose

5H_{2}O more, and on losing the last equivalent of water the

tension falls to 0·04 compared with water. It is clear that

the different molecules of water are held by an unequal force.

Out of the five molecules of water in copper sulphate the two

first are comparatively easily separated even at the ordinary

temperature (but only after several days in a desiccator,

according to Latchinoff); the next two are more difficultly

separated, and the last equivalent is retained even at 100°. This

is another indication of the capacity of CuSO_{4} to form three

hydrates, CuSO_{4}5H_{2}O, CuSO_{4}3H_{2}O, and CuSO_{4}H_{2}O.

The researches of Andreae on the tension of dissociation of

hydrated sulphate of copper showed (1891) the existence of three

provinces, characterised at a given temperature by a constant

tension: (1) between 3-5, (2) between 1-3, and lastly (3) between

0-1 molecule of water, which again confirms the existence of

three hydrates of the above composition for this salt.

[65 bis] Sodium acetate (C_{2}H_{3}O_{2}Na,3H_{2}O) melts at 58°, but

re-solidifies only on contact with a crystal, otherwise it

may remain liquid even at 0°, and may be used for obtaining a

constant temperature. According to Jeannel, the latent heat

of fusion is about 28 calories, and according to Pickering

the heat of solution 35 calories. When melted this salt boils

at 123°--that is, the tension of the vapour given off at that

temperature equals the atmospheric pressure.

It is most important to recognise in respect to the water of

crystallisation that its ratio to the quantity of the substance with

which it is combined is always a constant quantity. However often we

may prepare copper sulphate, we shall always find 36·14 p.c. of water

in its crystals, and these crystals always lose four-fifths of their

water at 100°, and one-fifth of the whole amount of the water contained

remains in the crystals at 100°, and is only expelled from them at a

temperature of about 240°. What has been said about crystals of copper

sulphate refers also to crystals of every other substance, which contain

water of crystallisation. It is impossible in any of these cases to

increase either the relative proportion of the salt or of the water,

without changing the homogeneity of the substance. If once a portion of

the water be lost--for instance, if once efflorescence takes place--a

mixture is obtained, and not a homogeneous substance, namely a mixture

of a substance deprived of water with a substance which has not yet lost

water--_i.e._ decomposition has already commenced. This constant ratio

is an example of the fact that in chemical compounds the quantity of

the component parts is quite definite; that is, it is an example of the

so-called _definite chemical compounds_. They may be distinguished from

solutions, and from all other so-called indefinite chemical compounds,

in that at least one, and sometimes both, of the component parts may be

added in a large quantity to an indefinite chemical compound, without

destroying its homogeneity, as in solutions, whilst it is impossible

to add any one of the component parts to a definite chemical compound

without destroying the homogeneity of the entire mass. Definite

chemical compounds only decompose at a certain rise in temperature;

on a lowering in temperature they do not, at least with very few

exceptions, yield their components like solutions which form ice or

compounds with water of crystallisation. This leads to the assumption

that solutions contain water as water,[66] although it may sometimes

be in a very small quantity. Therefore solutions which are capable

of solidifying completely (for instance, crystallo-hydrates capable

of melting) such as the compound of 84-1/2 parts of sulphuric acid,

H_{2}SO_{4}, with 15-1/2 parts of water, H_{2}O, or H_{2}SO_{4},H_{2}O

(or H_{4}SO_{5}), appear as true definite chemical compounds. If,

then, we imagine such a definite compound in a liquid state, and admit

that it partially decomposes in this state, separating water--not as

ice or vapour (for then the system would be heterogeneous, including

substances in different physical states), but in a liquid form, when

the system will be homogeneous--we shall form an idea of a solution as

an unstable, dissociating fluid state of equilibrium between water and

the substance dissolved. Moreover, it should be remarked that, judging

by experiment, many substances give with water not one but _diverse_

compounds,[67] which is seen in the capacity of one substance to form

with water many various _crystallo-hydrates_, or compounds with water

of crystallisation, showing diverse and independent properties. From

these considerations, _solutions[68] may be regarded as fluid, unstable,

definite chemical compounds in a state of dissociation_.[69]

[66] Such a phenomenon frequently presents itself in purely chemical

action. For instance, let a liquid substance _A_ give, with

another liquid substance _B_, under the conditions of an

experiment, a mere minute quantity of a solid or gaseous

substance _C_. This small quantity will separate out (pass

away from the sphere of action, as Berthollet expressed it),

and the remaining masses of _A_ and _B_ will again give _C_;

consequently, under these conditions action will go on to the

end. Such, it seems to me, is the action in solutions when they

yield ice or vapour indicating the presence of water.

[67] Certain substances are capable of forming together only one

compound, others several, and these of the most varied degrees

of stability. The compounds of water are instances of this

kind. In solutions the existence of several different definite

compounds must be acknowledged, but many of these have not

yet been obtained in a free state, and it may be that they

cannot be obtained in any other but a liquid form--that is,

dissolved; just as there are many undoubted definite compounds

which only exist in one physical state. Among the hydrates such

instances occur. The compound CO_{2} + 8H_{2}O (_see_ note

31), according to Wroblewski, only occurs in a solid form.

Hydrates like H_{2}S + 12H_{2}O (De Forcrand and Villard), HBr

+ H_{2}O (Roozeboom), can only be accepted on the basis of a

decrease of tension, but present themselves as very transient

substances, incapable of existing in a stable free state.

Even sulphuric acid, H_{2}SO_{4}, itself, which undoubtedly

is a definite compound, fumes in a liquid form, giving off

the anhydride, SO_{3}--that is, it exhibits a very unstable

equilibrium. The crystallo-hydrates of chlorine, Cl_{2} +

8H_{2}O, of hydrogen sulphide, H_{2}S + 12H_{2}O (it is formed

at 0°, and is completely decomposed at +1°, as then 1 vol. of

water only dissolves 4 vols. of hydrogen sulphide, while at 0·1°

it dissolves about 100 vols.), and of many other gases, are

instances of hydrates which are very unstable.

[68] Of such a kind are also other indefinite chemical compounds; for

example, metallic alloys. These are solid substances or

solidified solutions of metals. They also contain definite

compounds, and may contain an excess of one of the metals.

According to the experiments of Laurie (1888), the alloys of zinc

with copper in respect to the electro-motive force in galvanic

batteries behave just like zinc if the proportion of copper in

the alloy does not exceed a certain percentage--that is, until

a definite compound is attained--for in that case particles of

free zinc are present; but if a copper surface be taken, and it

be covered by only one-thousandth part of its area of zinc, then

only the zinc will act in a galvanic battery.

[69] According to the above supposition, the condition of solutions

in the sense of the kinetic hypothesis of matter (that is, on

the supposition of an internal motion of molecules and atoms)

may be represented in the following form:--In a homogeneous

liquid--for instance, water--the molecules occur in a certain

state of, although mobile, still stable, equilibrium. When a

substance _A_ dissolves in water, its molecules form with several

molecules of water, systems _An_H_{2}O, which are so unstable

that when surrounded by molecules of water they decompose and

re-form, so that _A_ passes from one mass of molecules of water

to another, and the molecules of water which were at this

moment in harmonious motion with _A_ in the form of the system

_An_H_{2}O, in the next instant may have already succeeded in

getting free. The addition of water or of molecules of _A_

may either only alter the number of free molecules, which in

their turn enter into systems _An_H_{2}O, or they may introduce

conditions for the possibility of building up new systems

_Am_H_{2}O, where _m_ is either greater or less than _n_. If

in the solution the relation of the molecules be the same as

in the system _Am_H_{2}O, then the addition of fresh molecules

of water or of _A_ would be followed by the formation of new

molecules _An_H_{2}O. The relative quantity, stability, and

composition of these systems or definite compounds will vary in

one or another solution. I adopted this view of solutions (1887,

Pickering subsequently put forward a similar view) after a most

intimate study of the variation of their specific gravities, to

which my book, cited in note 19, is devoted. Definite compounds,

_An__{1}H_{2}O and _Am__{1}H_{2}O, existing in a free--for

instance, solid--form, may in certain cases be held in solutions

in a dissociated state (although but partially); they are similar

in their structure to those definite substances which are formed

in solutions, but it is not necessary to assume that such

systems as Na_{2}SO_{4} + 10H_{2}O, or Na_{2}SO_{4} + 7H_{2}O,

or Na_{2}SO_{4}, are contained in solutions. The comparatively

more stable systems _An__{1}H_{2}O which exist in a free state

and change their physical state must present, although within

certain limits of temperature, an entirely harmonious kind of

motion of _A_ with _n__{1}H_{2}O; the property also and state of

systems _An_H_{2}O and _Am_H_{2}O, occurring in solutions, is

that they are in a liquid form, although partially dissociated.

Substances _A__{1}, which give solutions, are distinguished by

the fact that they can form such unstable systems _An_H_{2}O,

but besides them they can give other much more stable systems

_An__{1}H_{2}O. Thus ethylene, C_{2}H_{4}, in dissolving in

water, probably forms a system C_{2}H_{4}_n_H_{2}O, which easily

splits up into C_{2}H_{4} and H_{2}O, but it also gives the

system of alcohol, C_{2}H_{4},H_{2}O or C_{2}H_{6}O, which is

comparatively stable. Thus oxygen can dissolve in water, and it

can combine with it, forming peroxide of hydrogen. Turpentine,

C_{10}H_{16}, does not dissolve in water, but it combines with it

as a comparatively stable hydrate. In other words, the chemical

structure of hydrates, or of the definite compounds which are

contained in solutions, is distinguished not only by its original

peculiarities but also by a diversity of stability. A similar

structure to hydrates must be acknowledged in crystallo-hydrates.

On melting they give actual (real) solutions. As substances which

give crystallo-hydrates, like salts, are capable of forming a

number of diverse hydrates, and as the greater the number of

molecules of water (_n_) they (_An_H_{2}O) contain, the lower

is the temperature of their formation, and as the more easily

they decompose the more water they hold, therefore, in the first

place, the isolation of hydrates holding much water existing in

aqueous solutions may be soonest looked for at low temperatures

(although, perhaps, in certain cases they cannot exist in the

solid state); and, secondly, the stability also of such higher

hydrates will be at a minimum under the ordinary circumstances

of the occurrence of liquid water. Hence a further more detailed

investigation of cryohydrates may help to the elucidation of

the nature of solutions. But it may be foreseen that certain

cryohydrates will, like metallic alloys, present solidified

mixtures of ice with the salts themselves and their more stable

hydrates, and others will be definite compounds.

In regarding solutions from this point of view they come under the head

of those definite compounds with which chemistry is mainly concerned.[70]

[70] The above representation of solutions, &c., considering them as

a particular state of definite compounds, excludes the

independent existence of indefinite compounds; by this means that

unity of chemical conception is obtained which cannot be arrived

at by admitting the physico-mechanical conception of indefinite

compounds. The gradual transition from typical solutions (as

of gases in water, and of weak saline solutions) to sulphuric

acid, and from it and its definite, but yet unstable and liquid,

compounds, to clearly defined compounds, such as salts and their

crystallo-hydrates, is so imperceptible, that in denying that

solutions pertain to the number of definite but dissociating

compounds, we risk denying the definiteness of the atomic

composition of such substances as sulphuric acid or of molten

crystallo-hydrates. I repeat, however, that for the present the

theory of solutions cannot be considered as firmly established.

The above opinion about them is nothing more than a hypothesis

which endeavours to satisfy those comparatively limited data

which we have for the present about solutions, and of those

cases of their transition into definite compounds. By submitting

solutions to the Daltonic conception of atomism, I hope that we

may not only attain to a general harmonious chemical doctrine,

but also that new motives for investigation and research will

appear in the problem of solutions, which must either confirm the

proposed theory or replace it by another fuller and truer one;

and I for my part cannot consider this to be the case with any of

the other present doctrines of solutions (note 49).

We saw above that copper sulphate loses four-fifths of its water at

100° and the remainder at 240°. This means that there are two definite

compounds of water with the anhydrous salt. Washing soda or carbonate of

sodium, Na_{2}CO_{3} separates out as crystals, Na_{2}CO_{3},10H_{2}O,

containing 62·9 p.c. of water by weight, from its solutions at the

ordinary temperature. When a solution of the same salt deposits crystals

at a low temperature, about -20°, then these crystals contain 71·8

parts of water per 28·2 parts of anhydrous salt. Further, the crystals

are obtained together with ice, and are left behind when it melts. If

ordinary soda, with 62·9 p.c. of water, be cautiously melted in its

own water of crystallisation, there remains a salt, in a solid state,

containing only 14·5 p.c. of water, and a liquid is obtained which

contains the solution of a salt which separates out crystals at 34°,

which contain 46 p.c. of water and do not effloresce in air. Lastly,

if a supersaturated solution of soda be prepared, then at temperatures

below 8° it deposits crystals containing 54·3 p.c. of water. Thus as

many as five compounds of anhydrous soda with water are known; and

they are dissimilar in their properties and crystalline form, and even

in their solubility. It is to be observed that the greatest amount of

water in the crystals corresponds with a temperature of -20°, and the

smallest to the highest temperature. There is apparently no relation

between the above quantities of water and the salts, but this is only

because in each case the amount of water and anhydrous salt was given

in percentages; but if it be calculated for one and the same quantity

of anhydrous salt, or of water, a great regularity will be observed in

the amounts of the component parts in all these compounds. It appears

that for 106 parts of anhydrous salt in the crystals separated out at

-20° there are 270 parts of water; in the crystals obtained at 15° there

are 180 parts of water; in the crystals obtained from a supersaturated

solution 126 parts, in the crystals which separate out at 34°, 90

parts, and the crystals with the smallest amount of water, 18 parts.

On comparing these quantities of water it may easily be seen that they

are in simple proportion to each other, for they are all divisible by

18, and are in the ratio 15 : 10 : 7 : 5 : 1. Naturally, direct

experiment, however carefully it be conducted, is hampered with errors,

but taking these unavoidable experimental errors into consideration,

it will be seen that for a given quantity of an anhydrous substance

there occur, in several of its compounds with water, quantities of

water which are in very simple multiple proportion. This is observed

in, and is common to, all definite chemical compounds. This rule is

called _the law of multiple proportions_. It was discovered by Dalton,

and will be evolved in further detail subsequently in this work. For

the present we will only state that the law of definite composition

enables the composition of substances to be expressed by formulæ,

and the law of multiple proportions permits the application of whole

numbers as coefficients of the symbols of the elements in these formulæ.

Thus the formula Na_{2}CO_{3},10H_{2}O shows directly that in this

crystallo-hydrate there are 180 parts of water to 106 parts by weight of

the anhydrous salt, because the formula of soda, Na_{2}CO_{3}, directly

answers to a weight of 106, and the formula of water to 18 parts, by

weight, which are here taken 10 times.

In the above examples of the combinations of water, we saw the gradually

increasing intensity of the bond between water and a substance with

which it forms a homogeneous compound. There is a series of such

compounds with water, in which the water is held with very great force,

and is only given up at a very high temperature, and sometimes cannot

be separated by any degree of heat without the entire decomposition

of the substance. In these compounds there is generally no outward

sign whatever of their containing water. A perfectly new substance is

formed from an anhydrous substance and water, in which sometimes the

properties of neither one nor the other substance are observable. In

the majority of cases, a considerable amount of heat is evolved in the

formation of such compounds with water. Sometimes the heat evolved is

so intense that a red heat is produced and light is emitted. It is

hardly to be wondered at, after this, that stable compounds are formed

by such a combination. Their decomposition requires great heat; a large

amount of work is necessary to separate them into their component

parts. All such compounds are definite, and, generally, completely and

clearly definite. The number of such definite compounds with water or

_hydrates_, in the narrow sense of the word, is generally inconsiderable

for each anhydrous substance; in the greater number of cases, there is

formed only one such combination of a substance with water, one hydrate,

having so great a stability. The water contained in these compounds is

often called _water of constitution_--_i.e._ water which enters into the

structure or composition of the given substance. By this it is desired

to express, that in other cases the molecules of water are, as it were,

separate from the molecules of that substance with which it is combined.

It is supposed that in the formation of hydrates this water, even in

the smallest particles, forms one complete whole with the anhydrous

substance. Many examples of the formation of such hydrates might be

cited. The most familiar example in practice is the hydrate of lime,

or so-called 'slaked' lime. Lime is prepared by burning limestone, by

which the carbonic anhydride is expelled from it, and there remains

a white stony mass, which is dense, compact, and rather tenacious.

Lime is usually sold in this form, and bears the name of 'quick' or

'unslaked' lime. If water be poured over such lime, a great rise in

temperature is remarked either directly, or after a certain time. The

whole mass becomes hot, part of the water is evaporated, the stony mass

in absorbing water crumbles into powder, and if the water be taken in

sufficient quantity and the lime be pure and well burnt, not a particle

of the original stony mass is left--it all crumbles into powder. If

the water be in excess, then naturally a portion of it remains and

forms a solution. This process is called 'slaking' lime. Slaked lime is

used in practice in intermixture with sand as mortar. Slaked lime is a

definite hydrate of lime. If it is dried at 100° it retains 24·3 p.c.

of water. This water can only be expelled at a temperature above 400°,

and then quicklime is re-obtained. The heat evolved in the combination

of lime with water is so intense that it can set fire to wood, sulphur,

gunpowder, &c. Even on mixing lime with ice the temperature rises to

100°. If lime be moistened with a small quantity of water in the dark,

a luminous effect is observed. But, nevertheless, water may still be

separated from this hydrate.[71] If phosphorus be burnt in dry air, a

white substance called 'phosphoric anhydride' is obtained. It combines

with water with such energy, that the experiment must be conducted with

great caution. A red heat is produced in the formation of the compound,

and it is impossible to separate the water from the resultant hydrate

at any temperature. The hydrate formed by phosphoric anhydride is a

substance which is totally undecomposable into its original component

parts by the action of heat. Almost as energetic a combination occurs

when sulphuric anhydride, SO_{3}, combines with water, forming its

hydrate, sulphuric acid, H_{2}SO_{4}. In both cases definite compounds

are produced, but the latter substance, as a liquid, and capable of

decomposition by heat, forms an evident link with solutions. If 80 parts

of sulphuric anhydride retain 18 parts of water, this water cannot be

separated from the anhydride, even at a temperature of 300°. It is only

by the addition of phosphoric anhydride, or by a series of chemical

transformations, that this water can be separated from its compound

with sulphuric anhydride. Oil of vitriol, or sulphuric acid, is such a

compound. If a larger proportion of water be taken, it will combine

with the H_{2}SO_{4}; for instance, if 36 parts of water per 80 parts of

sulphuric anhydride be taken, a compound is formed which crystallises in

the cold, and melts at +8°, whilst oil of vitriol does not solidify even

at -30°. If still more water be taken, the oil of vitriol will dissolve

in the remaining quantity of water. An evolution of heat takes place,

not only on the addition of the water of constitution, but in a less

degree on further additions of water.[72] And therefore there is no

distinct boundary, but only a gradual transition, between those chemical

phenomena which are expressed in the formation of solutions and those

which take place in the formation of the most stable hydrates.[73]

[71] In combining with water one part by weight of lime evolves 245

units of heat. A high temperature is obtained, because the

specific heat of the resulting product is small. Sodium oxide,

Na_{2}O, in reacting on water, H_{2}O, and forming caustic soda

(sodium hydroxide), NaHO, evolves 552 units of heat for each part

by weight of sodium oxide.

[72] The diagram given in note 28 shows the evolution of heat on the

mixture of sulphuric acid, or monohydrate (H_{2}SO_{4}, _i.e._

SO_{3} + H_{2}O), with different quantities of water per 100

vols. of the resultant solution. Every 98 grams of sulphuric

acid (H_{2}SO_{4}) evolve, on the addition of 18 grams of water,

6,379 units of heat; with twice or three times the quantity of

water 9,418 and 11,137 units of heat, and with an infinitely

large quantity of water 17,860 units of heat, according to the

determinations of Thomsen. He also showed that when H_{2}SO_{4}

is formed from SO_{3} (= 80) and H_{2}O (= 18), 21,308 units

of heat are evolved per 98 parts by weight of the resultant

sulphuric acid.

[73] Thus, for different hydrates the stability with which they hold

water is very dissimilar. Certain hydrates hold water very

loosely, and in combining with it evolve little heat. From

other hydrates the water cannot be separated by any degree of

heat, even if they are formed from anhydrides (_i.e._ anhydrous

substances) and water with little evolution of heat; for

instance, acetic anhydride in combining with water evolves an

inconsiderable amount of heat, but the water cannot then be

expelled from it. If the hydrate (acetic acid) formed by this

combination be strongly heated it either volatilises without

change, or decomposes into new substances, but it does not again

yield the original substances--_i.e._, the anhydride and water,

at least in a liquid form. Here is an instance which gives the

reason for calling the water entering into the composition of the

hydrate, water of constitution. Such, for example, is the water

entering into the so-called caustic soda or sodium hydroxide

(_see_ note 71). But there are hydrates which easily part with

their water; yet this water cannot be considered as water of

crystallisation, not only because sometimes such hydrates have no

crystalline form, but also because, in perfectly analogous cases,

very stable hydrates are formed, which are capable of particular

kinds of chemical reactions, as we shall subsequently learn.

Such, for example, is the unstable hydrated oxide of copper,

which is not formed from water and oxide of copper, but which is

obtained just like far more stable hydrates, for example, the

hydrated oxide of barium BaH_{2}O_{2} equal to BaO + H_{2}O, by

the double decomposition of the solution of salts with alkalies.

In a word, there is no distinct boundary either between the water

of hydrates and of crystallisation, or between solution and

hydration.

It must be observed that in separating from an aqueous solution,

many substances, without having a crystalline form, hold water in

the same unstable state as in crystals; only this water cannot

be termed 'water of crystallisation' if the substance which

separates out has no crystalline form. The hydrates of alumina

and silica are examples of such unstable hydrates. If these

substances are separated from an aqueous solution by a chemical

process, then they always contain water. The formation of a new

chemical compound containing water is here particularly evident,

for alumina and silica in an anhydrous state have chemical

properties differing from those they show when combined with

water, and do not combine directly with it. The entire series of

colloids on separating from water form similar compounds with

it, which have the aspect of solid gelatinous substances. Water

is held in a considerable quantity in solidified glue or boiled

albumin. It cannot be expelled from them by pressure; hence,

in this case there has ensued some kind of combination of the

substance with water. This water, however, is easily separated on

drying; but not the whole of it, a portion being retained, and

this portion is considered to belong to the hydrate, although

in this case it is very difficult, if not impossible, to obtain

definite compounds. The absence of any distinct boundary lines

between solutions, crystallo-hydrates, and ordinary hydrates

above referred to, is very clearly seen in such examples.

We have thus considered many aspects and degrees of combination of

various substances with water, or instances of the compounds of water,

when it and other substances form new homogeneous substances, which

in this case will evidently be complex--_i.e._ made up of different

substances--and although they are homogeneous, yet it must be admitted

that in them there exist those component parts which entered into their

composition, inasmuch as these parts may be re-obtained from them. It

must not be imagined that water really exists in hydrate of lime, any

more than that ice or steam exists in water. When we say that water

occurs in the composition of a certain hydrate, we only wish to point

out that there are chemical transformations in which it is possible to

obtain that hydrate by means of water, and other transformations in

which this water may be separated out from the hydrate. This is all

simply expressed by the words, that water enters into the composition of

this hydrate. If a hydrate be formed by feeble bonds, and be decomposed

even at the ordinary temperature, and be a liquid, then the water

appears as one of the products of dissociation, and this gives an idea

of what solutions are, and forms the fundamental distinction between

them and other hydrates in which the water is combined with greater

stability.