Manchester Memoirs, Vol. Hi. (1908), No. 10- 29 



that the observations of pressure were consistent, on his 

 own principles, with an equilibrium involving simple binary 

 dissociation. 



But subsequent detailed investigations of gaseous 

 reactions, mainly by van't Hoff and his school, showed 

 that often, especially when more than two molecules were 

 concerned in the change, the results were abnormal both 

 as regards speed of reaction and ultimate equilibrium, — 

 and in fact indicated that the main part of the reaction 

 occurred not in the volume of the gaseous mixture, but in 

 contact with the walls of the containing vessel. 



A discussion of these difficulties in detail is hardly to 

 be ventured in this place, where so much of our know- 

 ledge as to the circumstances promoting or inhibiting 

 reaction in pure gases has been acquired. But just for 

 that reason it appears desirable to embrace the opportunity 

 to refer to a principle, which so far as my reading goes, 

 seems to have escaped attention.* In my copy of the 

 English edition of van't HofTs " Studies in Chemical 

 Dynamics" (1896) I find the following memorandum of 

 ten years ago. "It would be a corollary from this [estimate 

 of chances of encounter of molecules] that in gaseous 



* I find that Prof. II. B. Dixon has reasoned from the extreme rarity of 

 trimolecular encounters, in a paper on " The Mode of Formation of Carbonic 

 Acid in tlie Burning of Carbon Compounds," Trans. Chem. Soc, 1896 ((/. 

 P- 777)> which also brings to a focus many of the peculiarities of gaseous 

 reaction. In the same paper it was announced that the bimolecular reaction 

 of CO with N„0 is not excited by the electric spark when the gases are well 

 dried, though the mixture becomes explosive on the addition of water 

 vapour. The argument in the text need not imply that every bimolecular 

 reaction proceeds spontaneously ; in such a case as the oiie quoted the 

 ordinary explanation is appropriate, which regards a catalyst as opening a 

 path of transformation around an obstructing ridge on the surface of avail- 

 able energy, such as would present a barrier to direct combination. As in 

 electric phenomena, so in chemical change, the apparent anomalies of 

 reaction in pure gases seem destined to provide the clue towards deeper 

 insight, as, in fact. Lord Rayleigh pointed out long ago. 



