Conditions of Chemical Change. 173 



be or may not be, or whether certain " composite electrolytes" 

 may or may not be formed ; for such discussions seem to 

 reduce the fair field of experimental chemistry to the dry 

 and arid waste of academical metaphysics. 



(iii.) Period of Constancy. 



This period has been observed in most of the cases dis- 

 cussed in the previous section, and would follow as a neces- 

 sary consequence of two opposing factors, the one of accele- 

 ration, and the other of retardation due to the diminution of 

 the masses of reacting substances. Its duration is therefore 

 dependent upon the products of these factors ; and according 

 to experimental conditions it has been observed to last either 

 for the space of a few minutes or for several hours. 



Berthelot's experiments have also shown that the velocity 

 of propagation of explosive waves rapidly increases until a 

 constant maximum is reached, the value of which varies with 

 the nature of the exploding gases. Though the researches of 

 Dixon * have conclusively proved that this maximum for the 

 same pair of gases is dependent upon either the presence of 

 moisture or of an excess of one of the gases, or of a third 

 gas, whereby secondary reactions may ensue, yet, so far as I 

 am aware, the effects produced on the earlier period of acce- 

 leration have not been made the subject of any elaborate 

 series of experiments. I have therefore omitted their dis- 

 cussion in the preceding paragraph, though there can be but 

 little doubt that in these cases ■ also the formation of inter- 

 mediate compounds (carbonic oxide, for example, as the first 

 stage of the oxidation of carbon) would have a very important 

 bearing. 



(iv.) Period of Diminishing Velocity. 



It is only necessary to allude to the final stage, so cha- 

 racteristic of all save reversible chemical changes, and which 

 has so often been shown in the many elaborate memoirs 

 published in the last thirty or forty years to be a function of 

 mass, temperature, and other conditions. 



What is Chemical Change ? 



In the preceding paragraphs the course of chemical change 

 has been discussed ; we now pass on to the more difficult 

 problem concerning its nature. 



Some sixty years ago Faraday, impressed, doubtless, with 

 his wonderful experiments upon electrolysis, wrote |: — "The 



* Phil Trans. 1884, p. 617, and 1893 [A], pp. 97-188. 

 t • Experimental Researches.' 



