84

EFFECTS OF HEAT ON SOLUBILITY.

[55

heated, it becomes milky, and recovers its transparency as it cools. Calcic sulphate is also slightly more soluble in water at about 100° F. (38° C.) than it is in boiling water. A compound of lime and sugar, very soluble in cold water, is separated from the solution almost completely, if heated to boiling. But the most remarkable case of the kind occurs in sodic sulphate: this salt (the Glauber's salt of commerce) when crystallized requires about 10 times its weight of ice-cold water for solution, and its solubility increases rapidly as the temperature rises, until it reaches 33° (C. 91°4 F.): from this point until the solution boils, the solubility decreases; so that when a portion of the liquid saturated at 33° is heated more strongly without allowing the water to evaporate, hard gritty crystals are deposited, and the liquid when it boils retains only about four-fifths of the quantity which was dissolved at 33°. Sodic seleniate exhibits the same peculiarity; so also does ferrous sulphate, although in a less degree. These anomalous results may be partly explained by the consideration, that heat diminishes the force of adhesion as well as that of cohesion; generally speaking, cohesion is the more rapidly diminished of the two, although not uniformly so; and in the cases of which we are now speaking, it would appear that the adhesion to water decreases in a greater ratio than the cohesion of the saline particles. An important observation in relation to this subject has been made upon the composition of the salts just mentioned, which have been found to undergo a change at a temperature below that of boiling water: at the temperature of the air, these salts contain a certain quantity of water, known as water of crystallization; but this water is either wholly or partially expelled from the crystals at a boiling heat. The hard crystals of sodic sulphate which are deposited during the heating of the saturated solution contain no water. The supersaturation of saline solutions has been made the subject of an elaborate series of researches by Löwel, as well as by Gernez, Jeannel, Tomlinson, and De Coppet. In the course of these inquiries, it appeared that in many instances a salt which ordinarily crystallizes with a large proportion of water may be obtained in two or more different crystalline forms, in each of which it is generally united with a different quantity of water of crystallization. Sodic sulphate, for example, may be obtained in three different forms-viz., 1, the anhydrous salt (Na2SO4); 2, a hydrate with 7 H2O; and 3, a hydrate with 10 H2O. Each of these varieties has its specific solubility, which differs from the solubility of the other varieties of the same salt. It is, therefore, possible to have two or more solutions of the same salt at the

56.]

ADHESION BETWEEN LIQUIDS.

85

same temperature, each of which shall be saturated, and yet each of which shall contain, in equal weights, different quantities of the salt, when reduced to its anhydrous condition-the variation depending upon differences in the molecular constitution of the salt. Sodic carbonate (Na,CO), besides its ordinary form with 10 H2O, crystallizes in two different forms, each of which, singular to say, contains 7 H2O; but the solubility of these two varieties is different; and a similar observation has been made in the case of magnesic, zincic, ferrous, and cupric sulphates, and a few other salts. Salts which do not yield hydrated crystals never furnish supersaturated solutions.*

Diffusion of Liquids.

(56) Adhesion between Liquids. In the majority of instances adhesion between dissimilar liquids is very perfect; and, from the complete mobility of the particles, the two liquids become perfectly incorporated. A drop of alcohol or of oil of vitriol may be perfectly mixed with a quart or any other quantity of water; or a drop of water with a quart of alcohol or of oil of vitriol. There are instances, however, in which this perfect solution does not take place the cohesion of the particles of the two liquids may, at a certain point, balance their adhesion for each other, and they will become mutually saturated. For this reason, when ether is mixed with water by agitation, the greater part will separate on allowing the mixture to repose: the ether will have dissolved an eighth or a tenth of its bulk of water, and the water will have taken up about an equal proportion of ether. In a similar way the essential oils are soluble only to a very small extent in water; oil of peppermint, for instance, if agitated with water, and then left to rest, will, for the most part, separate, although a sufficient quantity will have been dissolved to communicate the flavour and odour of the essence to the water. In

Gernez enumerates about 24 other salts in addition to those mentioned above, as being able to furnish supersaturated solutions. Among these he includes potash and ammonia alum, and several double sulphates, sodic and ammonic phosphates; ammonic, strontic, and uranic nitrates; sodic, zincic, and plumbic acetates: potassic arseniate, ammonic oxalate, as well as sodic borate, thiosulphate, (hyposulphite), and citrate; together with citric and tartaric acids.

Such supersaturated solutions crystallize at once on the addition of a crystal of the salt itself, and often by contact with a glass or metallic rod, possibly owing to the action of the film of air which adheres to their surface. By a sufficient reduction of temperature, all these supersaturated solutions crystallize, some requiring a much lower temperature than others.

86

COHESION FIGURES.

[56.

other instances, the separation of the two liquids, as when oil and water are mingled, appears to be complete.

When chloroform is dropped into distilled water it sinks gradually, and the drops preserve their rounded outline: but if a drop or two of an alkaline solution be added, the surface of the chloroform becomes flattened; and it resumes its rounded character on again adding a few drops of an acid. This experiment shows what slight circumstances may modify the cohesive powers of a liquid, and its degree of adhesion to others; the adhesion of water to chloroform being increased by the addition of an alkali, and being again diminished by neutralizing the alkali.

(57) Cohesion Figures.-A curious illustration of the struggle between the forces of cohesion and adhesion is exhibited in the phenomena of cohesion figures, to which attention has been drawn by Tomlinson. (Phil. Mag. 1861 [4], xxii. 249, and 1862 [4], xxiii. 186.) These phenomena may be best examined by allowing a drop of some liquid sparingly soluble in water, such as kreasote, or one of the essential oils, to be deposited gently upon the surface of clean water in a wide glass vessel perfectly free from grease: the adhesion of the drop to the surface of the water will cause it to spread out into a film, but the cohesion

FIG. 26.

of the particles composing the drop immediately produces a reaction; if oil of lavender be used, the film opens in a number of places, producing a worm-eaten pattern, resembling that shown in fig. 26. The arms of this figure tend to gather themselves up into separate smaller drops, the adhesion of the water spreads them out again, then the cohesion of the oil reacts against this, and soon prevails: the consequence being the speedy formation of the original drop into a number of discs, with sharp, well-defined outlines and convex surfaces. This action is often so rapid that it requires a quick eye to follow all the changes.

Now it appears that every liquid

has its own peculiar figure, by which it, indeed, may often be easily distinguished from other liquids. These figures are usually more or less permanent, according as the liquid under trial is less or more soluble in water. The more soluble the liquid, the more quickly does the figure disappear. The figure of kreasote will last for five minutes; that of ether, or of alcohol, but for the fraction of a second. These figures are often extremely beautiful; they are usually altered when two liquids are mingled with each other; and, in many cases, a practised eye can, by the form of the figure produced, detect with certainty the nature of the substance which has been added to the original

*The best way to secure this is to rinse out a glass, to ordinary appearance clean, with a few drops of oil of vitriol, or with a strong solution of caustic potash, which must be allowed to flow over the entire surface, then to wash the glass out with abundance of clean water, not touching the inside either with the fingers or a cloth.

58.]

DIFFUSION OF LIQUIDS.

87

liquid. Indeed, it appears to be very probable that this fact may be extensively useful, as affording a rapid means of judging approximately of the purity of such bodies as the essential oils, many of which are often largely adulterated with the fixed oils, or still more often with oil of turpentine. Fig. 27

shows the appearance exhibited by kreasote; fig. 28, of pure ether; fig. 29, of alcohol. Indeed the films of fixed oils also have characters perfectly distinguishFIG. 30.

FIG. 29.

able. Sperm, fig. 30, and colza, fig. 31, each have their own cohesion figures; and Tomlinson considers that it would be easy for any one to detect a mixture of the two by the appearance of the film produced by a drop of such a mixture, the result being such as is shown in fig. 32.

FIG. 31.

FIG. 32.

(58) Diffusion of Liquids.-If two liquids susceptible of permanent admixture with each other, but of different densities, be placed in the same vessel, they will gradually become intermixed: -if, for instance, a tall jar be filled with the blue infusion of

88

FIG. 33.

DIFFUSION OF LIQUIDS.

[58.

litmus for about two-thirds of its capacity, and by means of a long funnel, as shown in fig. 33, a quantity of oil of vitriol be cautiously poured in, so as to occupy the lower portion of the jar, it will be found, after the lapse of two or three days, that the acid has become diffused through the liquid, which will consequently have assumed a red colour throughout. If watched at intervals, the progress of the mixture may be traced by the gradual change of colour from below upwards.

Graham in his researches upon this subject employed a very simple apparatus (fig. 34), for measuring the rate at which the diffusion takes place. His experiments were performed principally upon solutions of saline bodies, which were allowed to diffuse into water. A number of small phials of equal capacity (about 114.. or 4 oz. each), were prepared, with the necks ground to a uniform aperture of 31.5mm. or 124 inches in diameter; into these phials

FIG. 34.

the solutions for experiment were poured, to within 13mm. or half an inch of the top; the phials were then filled up with pure water. Thus charged, each phial was closed by a glass plate, and placed in a cylindrical vessel containing about 20 oz. (0.567 litre) of distilled water, the mouth of the solution phial being at least 25mm. below the surface of the water in the exterior vessel. The glass plate was

then cautiously removed.

The apparatus was

afterwards set aside in an undisturbed place,

and maintained at a steady temperature for several days. After a sufficient lapse of time, the mouth of the solution phial was again closed with a plate of glass, and the vessel withdrawn from the larger jar. The water in the outer jar was evaporated, and the salt that had passed into it was easily determined by weight. (Phil. Trans. 1850, 1 and 805, and 1851, 483.) This plan of procedure is distinguished by the term phial-diffusion.

In his more recent experiments Graham adopted the method of jar-diffusion-In these a cylindrical jar of about 16 centimetres in height and 10 centimetres in width was employed: 07 litre of distilled water was placed in the jar, and then by means of a pipette terminating in a very fine capillary tube, o 1 litre of the solution for diffusion was added slowly so as to form a stratum at the bottom of the jar. The jar was then set aside for some days in an apartment maintained