444 PRINCIPLES OF CHEMISTRY 



take place between CC1 4 and RBr n on the one hand, and CBr 4 ancf 

 RC1,, on the other. This case is convenient for investigation inas- k 

 much as the RCl n and RBr,, taken (such as BC1 3 , SiCl 4 , TiCl 4 , POC1 3 , 

 and SnCl 4 ) belong to those substances which are decomposed 

 by water, whilst CC1 4 and CBr 4 are not decomposed by water ; and 

 therefore, by heating, for instance, a mixture of CC1 4 -h SiBr 4 it te 

 possible to arrive at a conclusion as to the amount of interchange 

 by treating the product with water, which decomposes the SiBr 4 left* 

 unchanged and the SiCl 4 formed by the exchange, and therefore! 

 by determining the composition of the product acted on by the water 

 it is possible to form a conclusion as to the amount of decomposition. 

 The mixture was always formed with equivalent quantities for in- 

 stance, 4BC1 3 + 3CBr 4 . It appeared that there was no exchange 

 whatever on simple intermixture, but that it proceeded slowly, 

 when the mixture was heated (for example, with the mixture above 

 mentioned at 123 4-86 per cent, of Cl was replaced by Br after 14 days' 

 heating, and 6-83 per cent, after 28 days, and 10-12 per cent, when 

 heated at 150 for 60 days). A limit was always reached which 

 corresponded with that of the complemental system ; in the given 

 instance the system 4BBr 3 + 3CC1 4 . In this last 89-97 per cent, of 

 bromine in the BBr 3 was replaced by chlorine ; that is, there were- 

 obtained 89 '97 molecules of BC1 3 and there remained 10*02 molecules 

 of BBr 3 , and therefore the same state of equilibrium was reached as 

 that given by the system 4BC1 3 -f 3CBr 4 . Both systems gave one and 

 the same state of equilibrium at the limit, which is in agreement with 

 Berthollet's doctrine. 28 



although this reaction is one of substitution and not of combination. Consequently thek 

 phenomena studied by Ostwald depend but little on the measure of the reaction of tha| 

 salts, and more on the relations of the dissolved substances to water. In substitutions* 

 for instance 2NaNO 3 + H 2 S0 4 -2HN0 3 + Na?S0 4 , the volumes vary but slightly: in the 

 above example they are 2(88'8) + 53 3 and 2(41-2) -f'58'6 ; hence 181 volumes act, and 136 

 volumes are produced. It may be concluded, therefore, on the basis of what has been said, 

 that on taking water into consideration the phenomena studied by Thomson and Ostwald 

 are much more complex than they at first appear, and that this method can scarcely 

 lead to a correct interpretation as to the distribution of acids between bases. W0 

 may add that P. D. Chroustcheff (1890) introduced a new method for this class of 

 research, by investigating the electro-conductivity of solutions and their mixtures, anil 

 obtained remarkable results (for example, that hydrochloric acid almost entirely displace^ 

 formic acid and only of sulphuric acid), but details of these methods must be looked, 

 for in text-books of theoretical chemistry. 



28 G. G. Gustavson's researches, which were conducted in the laboratory of tu* 

 St. Petersburg University in 1871-72, are among the first in which the measure ol 

 the affinity of the elements for the halogens is recognised with perfect clearness in the limit 

 of substitution and in the rate of reaction. The researches conducted by A. L. Pdtilitzitt 

 (of which mention will be made in Chapter XI., Note 66) in the same laboratory touch on 

 another aspect of the same problem which has not yet .made tnuoti progress, notwith- 



