366 Mr. E. J. Mills on a Defect in the Theory of Saturation. 



combined with X, so long as they are interchangeable and of 

 the same equivalency. 



Such a doctrine of residual saturability appears of very consi- 

 derable interest. Based as it is on the well-known phenomena 

 of difference in affinity, it rests on a property of bodies still 

 unsubmitted to numerical measurement, but always received 

 as a fact. The idea of unequal affinity rests chiefly on results 

 derived from the decomposition of bodies — being thus comple- 

 mentary to the current doctrine of saturation, which more ex- 

 pressly leans on the facts of their synthesis. At the present 

 moment especially it is desirable that the importance of connect- 

 ing these should be taken into consideration. For unless we are 

 prepared to recognize this connexion, we are logically bound to 

 fall back on hydrogen-compounds only, and to content our- 

 selves, in the prospect of much inconvenience, with a dogmatic 

 announcement of merely partial value. But, on the other hand, 

 by accepting it, we are able to explain the formation of certain 

 compounds whose existence otherwise is quite anomalous. 



Thus chloride of silver is ordinarily considered a completely 

 saturated compound. Silver, indeed, being mono-equivalentic, 

 must, after combination with an equivalent of chlorine, be in- 

 capable of entering, according to the common view, into any- 

 further union whatsoever. This chloride contains no poly-equi- 

 valentic radical — that kind which is said to possess the power 

 of " accumulating " as its exclusive privilege. Yet the compound 



3AgCl + 2AgBr, 

 long since described, is perfectly definite; and, to judge from 

 its solubility in solutions of hydrochloric acid and of the chlo- 

 rides of potassium and sodium, chloride of silver also combines 

 with these bodies. The formula of chlorobromide of silver may 

 be expressed under the general one 



mX'W + nZ, 

 Z being a quite undetermined body. We see that the residual 

 saturability of three molecules of chloride of silver is capable of 

 causing them to unite with two of bromide of silver. This consi- 

 deration is in favour of the chloride being the more saturated 

 compound. 



The following examples may also be adduced : — 



AgBr + HBr, 

 Agl +2KI, 

 Agl +KI, 

 KI + 1, 



and also the pair 



AgCl + AgNO 3 , 

 AgT +AgN0 3 , 



