THEIR CAUSES. 293 



action ought to have retarded or prevented the oxi- 

 dation of the platinum or copper while they were in 

 contact with silver or zinc, but, as experience shows, 

 the influence of the opposite electrical conditions 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 

 together only by a feeble affinity. It is well known, 

 that there are chemical compounds of so unstable a 

 nature, that changes in temperature and electrical 

 condition, or even simple mechanical friction, or con- 

 tact with bodies of apparently totally indifferent na- 

 tures, cause such a disturbance in the attraction of 

 their constituents, that the latter enter into new 

 forms, without any of them combining with the act- 

 ing body. These compounds appear to stand but 

 just within the limits of chemical combination, and 

 agents exercise a powerful influence on them, which 

 are completely devoid of action on compounds of a 

 stronger affinity. Thus, by a slight increase of tem- 

 perature, the elements of hypochlorous acid* sep- 

 arate from one another with evolution of heat and 

 light ; chloride of nitrogen explodes by contact with 

 many bodies, which combine neither with chlorine 

 nor nitrogen at common temperatures ; and the con- 

 tact of any solid substance is sufficient to cause the 

 explosion of iodide of nitrogen, or fulminating silver. 



It has never been supposed that the causes of the 

 decomposition of these bodies should be ascribed to 

 a peculiar power, different from that which regulates 

 chemical affinity, — a power which mere contact with 

 the down of a feather is sufficient to set in activity, 

 and which, once in action, gives rise to the decom- 

 position. These substances have always been viewed 

 as chemical compounds of a very unstable nature, in 

 which the component parts are in a state of such 

 tension, that the least disturbance overcomes their 

 chemical affinity. They exist only by the vis inertice, 



* Formerly, protoxide of chlorine, 



25* 



