224 INDUCTIVE AFFINITY. 



evolved at the zincoid and sodium at the chloroid. But the 

 sodium having a strong affinity for oxygen reacts upon the water 

 at the pole, forming soda and liberating hydrogen, which there- 

 fore appear together ; while SO 4 having, as a high salt-radical, 

 a powerful affinity for hydrogen, likewise decomposes water, and 

 thus evolves oxygen, which, with a free acid, appears at the 

 zincoid. A solution of chloride of sodium is decomposed in the 

 same manner, its elements chlorine and sodium being attracted 

 to the zincoid and chloroid respectively, but neither of these 

 elements appearing as suchc Both decompose water and thus 

 produce oxygen with hydrochloric acid at the zincoid, and soda 

 with hydrogen at the chloroid. It has indeed been ascertained 

 that the polar influence which apparently effects two decom- 

 positions in these circumstances, namely, that of water into 

 oxygen and hydrogen, and of a salt into its acid and alkali, is no 

 more in quantity than is necessary to decompose one of these 

 bodies, the circulating power being measured by the quantity 

 of fused chloride of lead decomposed in another part of the 

 circuit (Daniell). There can be little doubt then that only one 

 binary compound is immediately decomposed, and that the two 

 sets of products which appear at the poles, are the results of 

 secondary decomposition. Indeed the decomposition of salts in 

 the voltaic circle affords considerable support to the binary 

 theory of these bodies (page 164). 



Secondary decompositions. The products of voltaic action are 

 frequently of that secondary character, the original products 

 being lost from their reaction upon the liquid in which they are 

 produced, or upon the substance of the metallic poles. Thus 

 salts of the vegetable acids often afford carbonic acid, and salts 

 of ammonia nitrogen, instead of oxygen, at the zincoid ; the 

 oxygen liberated having reacted upon the combustible constitu- 

 ents of these bodies. Nitrates, again, may afford nitrogen, or 

 nitric oxide, at the chloroid, in consequence of the oxidation of 

 the hydrogen evolved there. The nascent condition of the libe- 

 rated elements favours such secondary actions. When the 

 zincoid is composed of a positive metal, such as zinc itself or 

 copper, the chlorous element is absorbed there, combining with 

 the metaL The decomposition of a salt is also then much 

 easier, the action of the circle being greatly assisted by the 

 proper affinity of the matter of the zincoid for a chlorous body. 

 Insoluble sulphurets, chlorides and other compounds of a posi- 



