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GENERAL REMARKS ON DIFFUSION.

[70 a.

by capillary transpiration, nor by their proper diffusive movement, but only after liquefaction—such as the pores of wrought metals and the finest pores of graphite.'

This latter porosity Deville conceives to be an intermolecular porosity due entirely to dilatation. The intermolecular porosity of platinum and iron is not sufficient, he supposes, to admit any passage of gas at low temperatures, but is developed by the expansive agency of heat upon these metals, and becomes sensible in these particular cases about the temperature of ignition.

The absorption of a gas by a liquid or a colloid substance is not a purely physical effect. Some relation in composition is necessary; hence it is asked by Graham whether a similar analogy may not be looked for between hydrogen and colloid bodies of the metallic class.

The phenomena of diffusion in gases were viewed by Dalton as a necessary consequence of the self-repulsive property of the particles of gaseous bodies. He considered that each gas ultimately dilates until the whole space through which the diffusion occurs is filled with an atmosphere of that gas, of a density proportional to the quantity of the gas present. Observation shows that each gas becomes diffused through a limited space filled with any other gas, as it would be through a vacuum, the other gas only acting mechanically to retard the period at which such uniformity of diffusion is attained.

It has been remarked by Graham, that if this view were true, there should be, contrary to experience, a depression of temperature when two gases become intermixed. It does not, however, appear that this is a necessary consequence, since the particles of each gas may merely glide amongst those of the other kind, as the particles of water do amongst those of sand, the self-repulsion of the particles still being the power which determines the process of diffusion.

The phenomena of the diffusion of liquids seem, however, to be more easily reconciled with the supposition of a feeble superficial attraction between the particles of one liquid and those of another, and the supposition that an analogous attraction exists between the particles of one gas and those of a gas of different nature might sufficiently account for the process of intermixture in the case of elastic fluids.

It is to be borne in mind that in the intermixture of gases, the diffusion volume has no necessary relation to the chemical equivalent of the body. The ratios which have been observed are dependent upon the relative density of the gases compared,

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SEPARATION OF BODIES BY COLD OR HEAT.

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quite irrespective of the combining proportion. In liquids, a similar want of connexion between the chemical equivalent and the diffusion volume is observed; the relation in this case is a multiple of the absolute weight diffused.

(71) Separation of Bodies by Cold or Heat.-It often happens, where adhesion has proceeded so far as to produce the solution of a solid in a liquid, as in the cases just considered, that the chemist has occasion to destroy this adhesion, and to obtain one substance or both of them in a separate form. This separation is generally effected with the aid of heat. Depression of temperature will sometimes cause the cohesion of the particles of the solid to acquire the ascendancy over the force of adhesion. When, for example, brandy is exposed to intense cold, many degrees below that necessary to freeze water, the spirituous portion retains its liquid form, and separates from the aqueous part, which solidifies as ice. Indeed water, in the act of freezing, becomes. completely separated from everything which is previously held in solution. It is owing to the separation of air previously dissolved in the water, that ice so often presents a blebby, honeycombed appearance.

Faraday has shown that, even on a small scale, this complete separation of foreign matters from water may be easily effected by the process of freezing:-If sulphuric acid, or a strong solution of indigo, or one of common salt, be mixed with 90 or 100 times its volume of water, and this mixture be placed in a tube of about an inch in diameter, and immersed in a freezing mixture (175), at the same time that the separation of the foreign matter is mechanically facilitated by stirring the liquid round and round briskly and constantly with a feather, the sides of the tube will, in a few minutes, be lined with a coat of transparent, chemically pure ice, all the foreign matters having accumulated in the central portion, which still remains liquid.

It would appear that when the freezing of saline solutions takes place on a large scale, as in the neighbourhood of the Poles, the ice formed is by no means free from salt. Mr. Walker, who accompanied Sir L. M'Clintock in the Fox, made numerous observations on the freezing of sea water in the Arctic regions. He found that when the temperature fell below -2° C., ice began to form, at first as a thin pellicle, which gradually acquired a vertically striated appearance as it increased in thickness, plumose saline crystals separating upon the surface of the ice. Although he observed the formation of ice from sea water at all temperatures between -2° and -41° C., he never from this source could

126

SEPARATION OF BODIES BY COLD-CRYOHYDRATES.

[71.

The purest

obtain ice which on melting furnished fresh water. ice was that formed at the lowest temperature; but even that when melted furnished water of sp. gr. 1'005. He re-melted the ice from sea-water, and froze it again, repeating the operation several times upon the same portion of water, but never by this means succeeded in obtaining water of less density than 1002. Dr. Rae found that the ice of the previous winter, if above the level of the surrounding water, was usually fresh, probably from the draining away of the unsolidified brine previously entangled in the ice. (Proc. Phys. Soc. 1874, i. 14.) Mr. Buchanan made some experiments during the Antarctic cruise of the Challenger. He found that when pack ice was melted, the liquid which first flowed away contained more salt than the later portions, and that before the whole of the ice has disappeared, its temperature had risen from 1° to o° C. At this temperature almost fresh water was obtained. (Proc. Roy. Soc. 1876, xxiv. 609.) Dr. Guthrie has shown that when sea-water is frozen on a small scale by a freezing mixture, and the ice immediately strongly pressed between flannel, the ice contains only of the quantity of salts present in the original water. (Proc. Phys. Soc. 1874, i. 72.)

In studying the effect of cold on solutions, Dr. Guthrie (Proc. Phys. Soc. 1874, i. 53, 1875, ii. 1 and 53) has obtained some very remarkable results. The first series of experiments was conducted on solutions of sodic chloride in water. When a dilute solution of this salt is cooled below o° C. with constant stirring, ice is gradually deposited, the temperature at which the ice forms depending on the quantity of salt present. Thus a solution with 5254 per cent. of salt deposits ice at -3°4, with 10.508 per cent. at 7°7, with 15.762 per cent. at -12°4, and with 18-389 per cent. at -15°4. If, on the other hand, a saturated brine containing 26 27 per cent. of salt is cooled, a different result is observed; at -7° crystals of the hydrate of sodic chloride NaCl, 2 H2O are deposited, and this continues until the temperature reaches 22°, when the whole solidifies. By separating the crystals as they are formed until the temperature falls to -22°, a liquid is obtained possessing a composition approximately represented by the formula 2 NaCl,21H,O, and which solidifies at - 22°, the solid exhibiting a constant fusing point. This compound, or cryohydrate, may be, therefore, produced on cooling either a dilute solution, when ice first separates; or a strong solution, when salt first separates. The same treatment has been applied to a large number of other solutions with perfectly similar results, a series of cryohydrates being formed, cach possessing a

-

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CRYSTALLIZATION.

127

definite fusing point and composition. The fusing point of the cryohydrate is the minimum temperature obtainable by a freezing mixture of the salt and ice. In the case of salts which are more soluble in hot water than in cold, the curve indicating the solubility of the salt in water above o° is continuous with that representing the temperature at which the salt is deposited by cooling the solution below o°; it continues regular until the fusing point of the cryohydrate is reached, when it returns abruptly towards o°.

In like manner, gases may be in a great measure freed from condensible vapours by exposing them to a very low temperature. Air saturated with moisture may be rendered nearly dry by causing it to traverse a long tube, cooled down by immersion in a mixture of ice and salt.

Elevation of temperature is still more often resorted to for the separation of bodies in solution: when, for instance, a solution of common salt in water is exposed to heat, the repulsive power of this agent overcomes the cohesion of the water, as well as its adhesion to the salt; the water assumes the aëriform condition, passes off in steam, and leaves the salt behind in the solid state. This process is termed evaporation. It proceeds rapidly in shallow, open vessels, in which case the liquid escapes into the air. If it be necessary to preserve the solvent, the operation is conducted in a closed vessel, such as a retort, and connected with a suitable condensing apparatus, so as to effect a distillation of the liquid. The same process may be applied to effect a partial separation of liquids of different degrees of volatility; and spirit of wine is thus more or less perfectly separated from water.

§ IV. CRYSTALLIZATION.

(72) Modes of procuring Crystals.-It might be anticipated that when cohesion slowly recovers its ascendancy, this force would exert itself throughout the mass equally in all directions, and that a globular concretion would be the result, as when oil separates from mixture with dilute spirit of a specific gravity precisely equal to its own. The fact, however, is quite otherwise, for as a general rule cohesion is not exerted equally in all directions in solids. In the majority of instances, where solid bodies are allowed to separate slowly from their solutions, they are found to assume regular geometrical forms. Each substance has its own peculiar form. Such regular geometrical solids are termed crystals.

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MODE OF OBTAINING CRYSTALS.

[72.

By these differences in form, the materials which constitute the crystallized masses may often be distinguished from each other. For example, common salt crystallizes in cubes, alum in octohedra, saltpetre or nitre in six-sided prisms, Epsom salts in four-sided prisms, and so on. The more slowly and regularly the process is allowed to proceed, the larger and more regular are the crystals. The usual method of obtaining crystals is to form a strong solution of the salt in hot water, for most bodies are more freely soluble in water when it is at an elevated temperature than when cold; as the liquid cools, the cohesion of the salt resumes its ascendancy, and the crystals shoot through the liquid in this way crystals of nitre are easily procured.

It is not necessary, however, that the liquefaction should in all cases take place through the intervention of an indifferent liquid such as water: mere fusion of the substance, followed by slow cooling so as to allow it freely to obey the molecular attraction, is in many instances sufficient to produce crystals. If 2 or 3 kilogrammes of sulphur or of bismuth be fused in a crucible, and, after it has cooled sufficiently to become solid upon the surface, the crust be broken through and the yet liquid sulphur or bismuth be poured out, the inner surface of the solid portion will be found to be lined with prismatic transparent crystals of sul

FIG. 45.

phur, or brilliant hollow cubes of metallic bismuth. Water on solidifying often shoots into beautiful crystals, as may be seen in the forms of snow-flakes, fig. 45, which fall during a hard frost. The forms of these flakes are all derived from the six-sided plate. No. 1;

the separate crystals in the groups 2, 3, 4, 5, 6, 7, 8, all cross cach other at angles of 60° and 120°, though they vary in the complexity of their arrangement.

In the bowels of the earth, temperatures which man can hardly attain in his furnaces, have been acting for ages; processes of cooling of the most regular and gradual kind have been proceeding, and a great variety of combinations have been effected under the pressure of the superincumbent strata. By the combined operation of these causes many crystalline substances of mineral origin have been formed, which we have not succeeded in imitating, although a closer examination of the slags of our