168 Mr. V. H. Veley on the Phases and 



rather than in any other ; these molecules collide, the hy- 

 drogen and chlorine atoms change partners to produce hydro- 

 chloric acid. If it be further supposed that the external 

 temperature remains the same, and that the walls of the vessel 

 are absolutely non-conducting, then the heat evolved by the 

 combination of the hydrogen and chlorine would increase the 

 velocity of translation of the remaining molecules, and thereby 

 increase their tendency to combine, if the external cause of 

 the "set" continued. There would thus be an acceleration 

 of the combination up to the point at which there would be 

 so many particles of hydrochloric acid not only in the way 

 but also with an increasing velocity of translation. The 

 phenomena of acceleration discussed in the succeeding section 

 might be accounted for by dynamical considerations; but, 

 as will be seen, this is not the conclusion of the whole matter. 



However this may be, it is clear that when once a direction 

 has been given to the molecules or their constituents whereby 

 a chemical change is produced, they will continue to move in 

 the same direction, and the change, other external causes not 

 intervening, will proceed to its ultimate limit. The phe- 

 nomenon may be of the same kind as, or at least analogous to, 

 that observed by Faraday, viz., that when once an electric spark 

 has passed through a gas the passage of another immediately 

 afterwards is much facilitated, a fact confirmed by the expe- 

 riments of Hittorf and Schuster*. 



The theory of Clausius in a more or less modified form has 

 been almost universally adopted by chemists and physicists 

 alike. Armstrong f, however, practically rejects it on the 

 somewhat slender grounds that it affords no explanation of 

 the phenomena of electric conduction of certain silver salts in 

 the solid state, and of the variations in conductivity of mix- 

 tures of sulphuric acid and water. 



(ii.) Period of Inertness followed hy Acceleration. 



These phenomena were first noticed by Draper in the par- 

 ticular case of the reaction of hydrogen and chlorine gases 

 under the influence of sunlight, and traced by him to an 

 allotropic modification of the latter element {. Similar obser- 

 vations were made by Bunsen and Roscoe in their investi- 

 gations on the same subject, and the phenomenon was desig- 

 nated by them " Photochemical Induction " § ; a term which has 



* Cf. also the recent experiments of J. J. Thomson on the electrolysis 

 of steam (Proc. Roy. Soc. liii. pp. 90-110). 



t Proc. Roy. Soc. xl. p. 271. % Phil. Mag. [3] xxvii. p. 328. 



§ Phil. Trans. 1857, p. 381. 



