Conditions of Chemical Change. 175 



Again, the electric conductivity of hydrocarbons is for the 

 most part extremely small, yet in many cases they readily 

 enter into chemical change at ordinary temperature without 

 the intervention of extraneous energy. For example, many 

 of them combine directly with bromine with extreme violence, 

 yet it could hardly be supposed that a trace of hydrobromic 

 acid formed as a by-product in the midst of an insulator is 

 the determining cause. The reactions of the zinc and other 

 organo-metallic compounds upon oxygenated compounds pre- 

 sumably dry are also examples of changes wherein there is 

 as yet no evidence to show that they are dependent upon the 

 presence of an electrolyte, or that they are of the nature of 

 reversed electrolysis. Such examples might be multiplied, 

 and will readily occur to any student of chemistry. 



Lastly, is there any evidence to show that chemical 

 changes induced by finely divided platinum are conditioned 

 by the presence of an electrolyte ? or any proof that moisture, 

 or some other " tertium quid -' must be present ? or generally, 

 has any satisfactory explanation of the nature of surfaces 

 been given ? " Catalytic reactions are reactions which we 

 do not understand/' this has become a by-word and a re- 

 proach among chemists. Guesswork as to what may be is 

 of no avail ; we want to know what is. In any case there 

 seems nothing to warrant the exclusive selection of two par- 

 ticular forms of energy, namely the electric current and 

 chemical affinity, and to define absolutely the latter as the 

 reverse of the former. For with equal reason, and with as 

 fair a show of argument, heat and chemical energy might 

 likewise be defined as terms mutually reversible. Probably 

 the true position of the case is most exactly summed up by 

 the quotation from HelmhohVs writings given above, that 

 some of the very mightiest among chemical forces are of 

 electric origin. 



What is the cause of the commencement of Chemical Change f 



Of recent years the reactivity of substances has been con- 

 sidered to be conditioned by the degree of ionic dissociation, 

 the measure of which is expressed by the factor 



£C = 



U- 



5 



wherein u and u^ are the molecular conductivities at any 

 particular and at infinite dilution. This hypothesis has been 

 so widely accepted, not only as affording a convenient 



