ON ELECTEOLYSIS IN ITS PHYSICAL AND CHEMICAL BEABINGS. 377 



quantity of its conjugate body, and x the dissolved quantity of its opposite. The 

 ■coefficients of activity are S, a, (3, and y, respectively. 



Let us first examine the influence of pressure. 



If the body whose mass is c is a solid, one may reckon that pressure affects 

 nothing, and so — 



(45) A system heterogeneous xvith regard to a solid is independent of pressure . 



This proposition is in full agreement with fact, according to the researches of 

 M. Bunsen (Liebig's Ann., T. 6.5, p. 81, 1848). 



If, on the other hand, the c body is a gas, c will be approximately proportional 

 to the pressure of the gas in question on the surface of the solvent. So if this pres- 

 sure is zero, x = 0. This case is realised if one lets the gas escape from the solvent 

 as fast as it is produced. If the pressure increases, c increases also. For this 

 reason the opposite bodies (whose masses are x ^ and x y) will increase, and the con- 

 jugate bodies will diminish. Such a phenomenon has been observed in a lot of cases. 



If, for instance, one of the opposite bodies is also a gas, as (for instance) when 

 HjS acts on an alkaline carbonate, one has the equation — 



0-—x) a ,cd = c'3. Xy. 



By giving to the pressure of IIoS a sensible value, and by separating the carbonic 

 acid which is liberated [e.g., by means of a continuous stream of HjS], one finally 

 -arrives at the result that c', and therefore also 1 — .r, are zero, i.e., that all the 

 carbonate will be transformed into sulphide. Vice versa, by leaving to c' a sensible 

 value, and decreasing c, one could displace the H„S of an alkaline sulphide by CO.,. 



Nevertheless, in the majority of cases the mass of gas dissolved is small in 

 comparison with other matter dissolved. In this case x, that is the quantity of 

 opposite bodies formed, will be insignificant, so that the influence of the gas on the 

 system is not notable. Thus, we cannot in general observe changes of equihbrium 

 produced by the presence of gas, unless the dissolved quantity of the opposite body 

 is very small. This case is realised if the body is either gaseous or but slightly 

 soluble, and it has been considered. But in the second case, one could perceive 

 the influence of the gas, for the products due to its presence would precipitate them- 

 selves and disappear from the system. The precipitations of metallic salts by H^S 

 is a well-known example. 



An influence, analogous to this of pressure, is exerted by temperature if the 

 body with regard to which the system is heterogeneous is solid. The solubility of 

 solids increases with heating. So c and x increase, while 1—x, the quantity of 

 conjugate body, diminishes. This conclusion has been verified by numerous 

 experiments. One finds many in the works of M. Ostwald, who has studied the 

 phenomena accompanying the solution of some oxalates (CaCjO^^ or ZnOjO^) by an 

 acid. M. Ostwald expresses the said conclusion thus : — 



(46) The effect of a rise of temperature on the quantities dissolved hy a 



given acid at constant dilution is always to increase them. 



Another way of increasing the dissolved quantity, c, of a slightly soluble body 

 is to increase the volume of the solution (i.e., to add solvent). In this case one 

 increases fi and y also, as well as a. At the same time solubility can be diminished 

 hy diminution of concentration, as we shall soon see. But in most cases these 

 disturbing influences cannot compensate the increase of c by augment of volume. 

 So — 



(47) An increase of solvent increases the body opposite to, and diminishes 



the body conjugate to, the solid 2vith regard to which the system is 

 heterogeneous (if the perturbing influences are not too great). — 

 Ostwald, J.fiirpract. Vhemie (1881), T. 23, p. 517, and T. 24, p. 486. 



[The author then goes on to consider these ' perturbing causes.' He quotes 

 examples where the solubility of one substance in water has been shown to be 

 increased by the addition of some other salt. He explains this phenomenon on 

 his theory, and holds that the theory of Guldberg and Waage is insufficient to 

 -account for it, and, moreover, that it is not in agreement with the numerical 

 results of Ostwald.] 



