376 EEPORT— 1886. 



On the other hand, for HCl : HNO3, Q is almost constant, only varying from I'OO to 

 1-02 between 0° and 60°. 



A great part of the cited observations (those on HjSOJ have been explained 

 by Ostwald as due to the formation of acid salts at moderate dilution. We do 

 not wish to deny this as a vera causa. We have already, in § 10, shown that 

 the coefficient of activity diminishes for this reason. So naturally Q diminishes 

 too. Nevertheless, it is difficult to explain thus the variation of Q with tempera- 

 ture. It seems much more reasonable to admit the explanation given above. 



Finally, we will consider what ought to happen to a salt solution if its tempera- 

 ture rise. For this it is necessary to observe -that the temperature coefficient of 

 a concentrated hydrate is much greater than that of a weak solution. Thus, accord- 

 ing to M. Kohlrausch — ^ 



For sulphuric acid, at 99'4 per cent, concentration, b - '0426 



„ 7-73 „ „ 5 = -0121 



For phosphoric acid, „ 87'1 „ „ 6 = •0374 



„ 4-92 „ „ 6 = -0099 



For tartaric acid, „ 49-53 „ „ 6 = -0263 



„ 4-95 „ „ 6 = -0186 



For caustic potash, „ 41-7 „ „ b = -0282 



„ 4-19 „ „ 6 = -0188 



For soda, „ 1272 „ „ 6 = -0710 



„ 2-61 „ „ 6 = -0195 



For ammonia, „ 16-16 „ „ b = -0303 



„ 0-1 „ „ 6 = -0247 



As this rule is without known exception, it seems permissible to attribute to water, 

 which is also a hydrate, a coefficient much surpassing the coefficients of dilute 

 compounds (which latter coefficients are almost equal among themselves). Some 

 figures of Ayrton and Perry {Proc. Phys. 80c., II. 178 [1877]) tend to confirm this. 

 Similarly, the coefficient of a salt solution is sensibly greater than that of a hydi-ate. 

 Looking, then, at the equation of equilibrium of a saline solution — 



(,p + x)xab{l + b,t) (1 + b^t) = (1-xy ^y(l + bj) (1 + b^t), 



we see that 6, and b^ are bigger than b.^ and b^, and therefore that the value of 

 l—.r obtained from it will increase with ^. But 1 — .r signifies the amount of salt 

 decomposed, so — 



(44) The quantity of salt decomposed in a solution is increased if the tempera- 

 ture of the solution rises. 



This will be true also for other electrolytic solvents, according to the law of 

 Hittorf (see § 4). One may imagine that this important law can be otherwise 

 deduced from the idea of a dissociation of salt molecules. True, physics show 

 that at high temperatures the movements of molecules are more vigorous. But 

 this does not point out why water then attacks salts more strongly. One often 

 slurs over these difficulties by saying that heating (as well as dilution) brings to 

 the solution a foreign energy opposed to chemical action. But what then is the 

 import of chemical laws if they can be annulled by causes which never disappear 

 completely, and of which one does not know how to calculate the effect ? 



The above law is so generally adopted, and verified by a crowd of cases so 

 enormous, that it is not necessary to cite any of them. 



§ 13. Consequences of the variation of the coeflBcient of activity 

 and of solubility in heterogeneous systems. 

 The influence of the variation of the coefficient of activity is much greater for 

 heterogeneous systems in equilibrium than for homogeneous ones. The equation 

 (2a) has the form — 



(l-.r)a.eS = .r')3y, 



where c is the quantity dissolved of a slightly soluble or gaseous body, (1 — x') the 

 ' Pogg. Ann., T. 159, p. 233 ; and Wicd. Ann., T. 6, p. 1 (1876 and l%n\ 



