EXAMPLES OF THE EXPLANATION OF LAWS. 563 



num be treated with nitric acid, the acid does not, as 

 might naturally be expected, separate the two metals, 

 dissolving the silver, and leaving the platinum ; it 

 dissolves both : the platinum as well as the silver 

 becomes oxidized, and in that state combines with the 

 undecomposed portion of the acid. In like manner, 

 " 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." These phenomena cannot be 

 explained by the laws of what is termed chemical 

 affinity. They point to a peculiar law, by which the 

 oxidation which one body suffers, causes another, in 

 contact with it, to submit to the same change. And 

 not only chemical composition, but chemical decom- 

 position, is capable of being similarly propagated. The 

 peroxide of hydrogen, a compound formed by hydrogen 

 with a greater amount of oxygen than the quantity 

 necessary to form water, is held together by a chemical 

 attraction of so weak a nature, that the slightest cir- 

 cumstance is sufficient to decompose it ; and it even, 

 though very slowly, gives off oxygen and is reduced 

 to water spontaneously (being, I presume, decom- 

 posed by the tendency of its oxygen to absorb heat 

 and assume the gaseous state). Now it has been 

 observed, that if this decomposition of the peroxide 

 of hydrogen takes place in contact with some metallic 

 oxides, as those of silver, and the peroxides of lead 

 and manganese, it superinduces a corresponding che- 

 mical action upon those substances ; they also give 

 forth the whole or a portion of their oxygen, and are 

 reduced to the metal or to the protoxide; although they 

 do not undergo this change spontaneously, and there 

 is no chemical affinity at work to make them do so. 

 Other similar phenomena are mentioned by Dr. Liebig. 



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