DECOMPOSITION OF WATER BY HEAT. 331 



changing our views regarding them, I cannot at present see 

 that the title of my paper could be altered without being open 

 to greater objections. I am of this opinion, not so much 

 because other bodies than platinum will produce the effect, as 

 I shall presently show, nor from the fact that the electrical 

 spark will decompose aqueous vapour, though these are argu- 

 ments in its favour, but from the following considerations. 

 The catalytic action of platinum will induce or enable combi- 

 nation to take place where there is already a strong affinity 

 or tendency to combine, as with mixed oxygen and hydrogen 

 gases ; it will also induce decomposition where the affinities 

 are extremely weak, or in a state of unstable equilibrium, 

 as in Thenard's peroxide of hydrogen ; again, where there 

 are nicely-balanced compound affinities, it may change the 

 chemical arrangement of the constituents of a compound, but 

 I do not know of any case in which a powerful chemical 

 affinity can be overcome by catalytic action ; to effect this we 

 require some natural force of greater intensity than that to be 

 overcome. We might as well say that the platinum electrodes 

 of a voltaic battery decompose water, as to say that platinum 

 decomposes it in the case in question ; there the force of 

 electricity acts only by means of matter, and matter of a 

 peculiar description ; its action also is only perceptible at the 

 surface of this matter. I seek to use the expression in my 

 title with reference to heat in a similar sense to that in which 

 we use similar terms with reference to electricity, i.e. to regard 

 heat as the immediate dynamic force which overcomes the 

 affinity ; thus, as we say when employing the voltaic battery, 

 that we decompose water by electricity, so here we should say 

 that we decompose it by heat. 



If it be said that heat so weakens or antagonises the 

 affinity of the elements of water as to enable catalytic action 

 to separate them, this amounts to the same theory, as heat is 

 then regarded as the antagonising force, and in this case the 

 action, both thermic and catalytic, is the reverse of the normal 

 action. I have thought it desirable shortly to discuss this 

 question, as likely to lead to farther investigation, though I 

 have been somewhat embarrassed by the want of definite 

 meaning in the term catalysis ; I must plead guilty to have 



