156 ATTRACTION. 



So, in decomposing compound bodies, either by affinity, or heat, 

 the last portions are sometimes separated with much greater diffi- 

 culty than the first ; thus the black oxide of manganese easily gives 

 up one proportion of oxygen by a red heat, but no degree of heat 

 can expel the whole. In the same manner, the last portions of car- 

 bonic acid are expelled from carbonate of lime, with great difficulty 

 the first with ease. 



To effect complete decompositions, also, it is sometimes necessa- 

 ry to employ large quantities of the decomposing substances, as in 

 precipitating a metallic oxide from its union with an acid, and in de- 

 composing various salts by acids, as the nitrate of potash by sulphu- 

 ric acid. 



Partial decompositions are produced also by the exertion of a 

 weaker affinity, if it is aided by a larger quantity of matter, as in the 

 case of muriate of soda and oxide of lead. From these, and other 

 similar facts, the distinguished chemist Berthollet drew the conclu- 

 sion " that affinity is modified by quantity of matter, or that the 

 chemical action of a body is exerted in the ratio of its affinity and 

 quantity of matter, and he endeavored to establish it as a law, apply- 

 ing to all cases of chemical combination." (Murray.} He sup- 

 posed also that " w T hen two substances are in competition to com- 

 bine with a third, each of them obtains a degree of saturation pro- 

 portionate to its affinity multiplied by its quantity ; a product which 

 he denominates mass." (Ure.) 



Berthollet supposed that the tables of affinity expressed merely 

 the actual order of decomposition, as influenced, not only by affinity, 

 but by quantity of matter, and many other circumstances, and that 

 there was no such thing as a settled force of affinity, between differ- 

 ent substances. Berthollet contended also, that in proportion as it 

 requires more of a particular base to saturate a given acid, the less 

 is the affinity between that acid and the base. 



But we will not occupy time with views, which however ingenious 

 and ably supported, appear not to be universally tenable. Many of 

 tire facts adduced in support of them, can be explained in other 

 ways, and the well established doctrine of definite proportions could 

 not be true, were there no exact force of affinity, independent of 

 quantity of matter. Still, quantity of matter does undoubtedly ope- 

 rate in many cases, to a certain extent, and " although incompetent 

 to counteract direct and strong affinities, or to affect the combination 

 of bodies which are disposed to unite in definite proportions, its in- 

 fluence may be clearly traced in a number of instances, where it 

 modifies weaker attractions, and perhaps decides the result, when 

 opposite affinities are nearly balanced."* (Murray.) 



* See Prof. E. Mitchell's paper on the effect of quantity. Jim. Jour. Vol. XVI. 

 p. 234. 



