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respect to each, it contains 5‘2 per cent, of potassium, and 10-4 per cent, of copper sulphate. But if, instead of being in equilibrium with the single salts, it is in contact only with the double salt K2S04.CuS04.6H20, it contains 11'14 per cent, of the salt in solution. Lastly, the solution may be saturated with respect to the double salt and either of the single salts. At the same temperature, therefore, four different conditions of saturation equilibrium are possible, the condition which obtains depending solely on the salts in contact with action, developed by Harcourt and Esson (1866), and the solution. If the temperature be also varied, the conGuldberg and Waage (1867'). The discussion _ of the ditions become still more complex, owing not only to the facts relating to the saturation of solvents with dissolved changes in solubility, but also because alterations maymatter is, however, now recognized as lying within a take place in the degree of hydration and in the compositotally distinct province, and as involving the consideration tion of the double salts. Thus, in the case of potassium of heterogeneous systems. Neglect of this fact has proved and magnesium sulphates in solution at different tema constant source of error in the past, while its recognition peratures, the conditions under which equilibrium obhas led within recent years to far-reaching and fruitful tains vary according as they lead to the separation of •generalizations. The subject has received very full treat- one or other of the compounds H20 (as ice), K2S04, ment at the hands of van’t Hoff in his Lectures on Theoreti- MgS04.6H.,0, MgS04.7H00, K2S04.MgS04.6Ho0, or . cal and Physical Chemistry. Formerly it was the custom K0S04. MgS04.4H00. “it should be pointed out that the great advance made m to talk of a solution as being saturated with a certain dissolved material, but it is now recognized that it must recent years in studying problems such as those just rebe spoken of as saturated ivith respect to a particular ferred to is mainly due to the application by material. The distinction lies in the fact, not formerly Willard Gibbs of the second law of thermo- ?/****“' appreciated, that for a solution to become saturated it dynamics to the study of the physical equilibria efficients. must be in contact with undissolved material. For underlying chemical changes. It is characterexample, on shaking water with excess of sodium chloride istic of the results arrived at with the aid of this law,, that at 14°, each 100 parts of water dissolve 35'87 parts of they deal only with the difference in condition obtaining sodium chloride, and being in contact with the solid salt, between the various parts of a system, and their validity the solution is both in equilibrium with, and saturated does not depend upon the differences being of any particuwith respect to, the solid. If the solution be separated lar chemical or physical type. The principles governing from the solid it can no longer be described as a saturated heterogeneous equilibrium have been formally expressed This rule, salt solution, because it is no longer in contact with, by Willard Gibbs in the so-called phase rule. however, is merely a convenient method of classifying the and consequently not in equilibrium with solid sodium chloride. This modern method of regarding a state of various cases of equilibrium which may arise, and in itself saturation as a condition of equilibrium in a heterogeneous is of little use in diagnosing the conditions which hold in system, i.e., one in which there are at least two regions of any particular case. Nernst has applied the principles different composition or concentration, leads immediately enunciated by Willard Gibbs with signal success to the to a rational explanation of the phenomena of super- consideration of the equilibrium established when a soluble saturation which long perplexed chemists. Two cases substance is distributed between two practically immiscible may be cited in illustration. It is possible to dis- solvents in contact. For example—on shaking small Super-. solve 40 parts of sodium sulphate in 100 parts of quantities of succinic acid with a mixture of ether and saturation. water at an(j on cautiously cooling the soluwater, the ratio of the amount of acid taken up by the '-'2 tion to 10° no separation takes place, although at 10°, in contact with the salt, water will only take up 9 parts of sodium unit volume of ether, C4, and of water, C2, respectively, sulphate per 100 parts. The solution containing 40 parts of is found to be independent of the total concentration. the salt at 10° is commonly said to be supersaturated, This is known as the law of constant distribution cobut it cannot even be said to be saturated in the modern efficient. In the form given, it is obeyed only when the sense of the term. On dropping into it a fragment of the dissolved material has the same molecular weight in decahydrated salt, this salt at once crystallizes out, heat both solvents. On taking benzene, water, and benzoic being evolved, and when, eventually, the temperature is re- acid, which consists of double molecules in benzene soluduced to 10°, the solution contains only 9 parts of the tion, the distribution of this acid takes place in such a salt in 100 of water; this is because equilibrium has manner that ■% is constant, ^ being the amount of acid been established between solid and solution. Similarly, 2 the solubility curve of sodium sulphate in water exhibits a taken up by the unit volume of water, and C2 that taken up by the benzene. The study of distribution coefficients, break at 33°. The existence of this was long^ regarded therefore, affords valuable information as to the molecular as anomalous, but it occurs because below 33° the solid in contact with the saturated solution is the compound weights of substances in different liquid solvents.. Its Na9S04.10H20, whilst above 33° it is the anhydrous application to electrolytes in water and in non-dissociating sulphate. The hydrated salt decomposes at 33°, losing its liquids has furnished evidence supporting the electrolytic water, and this temperature is therefore termed the transi- dissociation hypothesis. The method may even be apphet tion temperature. At the transition temperature the solu- to solids. In the case of isomorphous mixtures it affords tion is in contact and in equilibrium with both hydrated evidence that these may properly be regarded as solid solutions of the one substance in the substance with which and anhydrous salt. In the case of two salts capable of forming a double salt it is isomorphous. Thus thallium and potassium chlorates the conditions are somewhat more complex. The results are isomorphous, and on crystallizing thallium chlorate from obtained by Trevor (at 25°) with potassium and copper an aqueous solution containing a small proportion or sulphates may be quoted in illustration. If the solution potassium chlorate crystals of the former containing a smaf be in contact with the two salts, and be saturated with proportion of the potassium salt are deposited. Judging derivable from the salt should be produced. The fact that solutions of cyanides smell of hydrogen cyanide is traceable to this cause. Another illustration is the liberation of the very weak base ammonia on boiling solutions of ammonium salts. The study of ionic or electrolytic dissociation, and of the properties of solutions generally, in the main, involves the investigation of homogeneous equilibria, Equilibrium t]ie principles governing which have long been ,a solution. £am-par £n connexion with the laws of mass