278 INDUCTION. 



less do so if placed in contact with some other body which is in the 

 act of yielding to the same force. Nitric acid, for example, does not 

 dissolve pure platinum, which may " be boiled with this acid without 

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

 no longer reflects light." But the same acid easily dissolves silver. 

 Now if an alloy of silver and platinum be treated with nitric acid, the 

 acid does not, as might naturally be expected, separate the two metals, 

 dissohang the silver, and leaving the platinum ; it dissolves both : the 

 platinum as well as the silver becomes oxidized, and in that state com- 

 bines with the undecomposed portion of the acid. In like manner, 

 "copper does not decompose water, even when boiled in dilute sul- 

 phuric acid, but an alloy of copper, zinc, and nickel, dissolves easily 

 in this acid with evolution of hydrogen gas." These phenomena can- 

 not 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 subrriit to the same change. 

 And not only chemical composition, but chemical decomposition, 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 circumstance is suffi- 

 cient to decompose it; and it even, though very slowly, gives oft' oxygen 

 and is reduced to water spontaneously (being, I presume, decomposed 

 by the tendency of its oxygen to absorb heat and assume the gaseous 

 state). Now it has been obser^•ed, 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 super- 

 induces a coiTesponding chemical 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. "Now no other explanation," he observes, "of these phe- 

 nomena can be given, than that a body in the act of combination or 

 decomposition enables another body, with which it is in contact, to 

 enter into the same state." 



Here, therefore, is a law of nature of great simplicity, but which, 

 owing to the extremely special and limited character of the phenomena 

 in which alone it can be detected experimentally (because in them 

 alone its results are not intermixed and blended with those of other 

 laws), had been very little recognized by chemists, and no one could 

 have ventured, on experimental evidence, to affirm it as a law common 

 to all chemical action ; owing to the impossibility of a rigorous employ- 

 ment of the Method of Difference where the properties of different kinds 

 of substance are involved, an impossibility which we noticed and char- 

 acterized in a previous chapter.* Now this extremely special and ap- 

 parently precarious generalization has, in the hands of Liebig, been 

 converted by a masterly employment of the Deductive Method, into a 

 law pervading all nature, in the same way as gravitation assumed that 

 character in the hands of Newton ; and has been found to explain, in 

 the most unexpected manner, numerous detached generalizations of a 



* Supra, p. 239, 



