THEIR CAUSE. 221 



by it, even when in a state of such fine division, 

 that it no longer reflects light (black spongy 

 platinum). But an alloy of silver and platinum 

 dissolves with great ease in nitric acid ; the oxida- 

 tion which the silver suffers, causes the platinum to 

 submit to the same change ; or, in other words, 

 the latter body from its contact with the oxidizing 

 silver, acquires the property of decomposing nitric 

 acid. 



Copper does not decompose water, even when 

 boiled in dilute sulphuric acid, but an alloy of 

 copper, zinc, and nickel, dissolves easily in this acid 

 with evolution of hydrogen gas. 



Tin decomposes nitric acid with great facility, 

 but water with difficulty ; and yet, when tin is dis- 

 solved in nitric acid, hydrogen is evolved at the 

 same time, from a decomposition of the water con- 

 tained in the acid, and ammonia is formed in 

 addition to oxide of tin. 



In the examples here given, the only combina- 

 tion or decomposition which can be explained by 

 chemical affinity is the last. In the other cases, 

 electrical action ought to have retarded or prevented 

 the oxidation of the platinum or copper while they 

 were in contact with silver or zinc, but, as experience 

 shows, the influence of the opposite electrical con- 

 ditions is more than counterbalanced by chemical 

 actions. 



The same phenomena are seen in a less dubious 

 form in compounds, the elements of which are held 



