Conditions of Molecular Action. 369 



surfaces more or less of any vapour or gas to which they are 

 exposed*. 



It is not difficult to understand why the mechanical condition 

 of the solid should materially affect the result. Since molecular 

 attraction is independent of the mass, and since its power is only 

 exerted at very small distances, it is evident that molecular at- 

 traction (strictly so called) is only exerted upon foreign sub- 

 stances at the surfaces of the active body (catalyte). The second 

 stratum of molecules will scarcely, if at all, affect substances 

 lying on the surface. It follows, therefore, that a large extent 

 of surface is an important assistance to molecular (catalytic) ac- 

 tions : thus spongy or laminated bodies act more readily than 

 dense solids. Finely divided or pulverulent substances enjoy 

 this advantage to the utmost ; but they are under favourable cir- 

 cumstances from another cause — namely, that they present to the 

 substances submitted to their influence an infinite number of 

 points. 



Action of Points. — When a particle of matter rests upon a 

 surface, it comes within the range of the molecular attractions 

 of several of the particles composing that surface. It is affected 

 not only by the molecule upon which it immediately rests, but 

 also by those closely surrounding it ; and the influence of these 

 last-mentioned molecules perturbs the action of the one support- 

 ing molecule, and prevents it from exerting its full power. 

 When, however, this molecule forms a point protruding from the 

 surrounding surface, it is removed to that extent from the range 

 of the disturbing forces, and its own peculiar energy is brought 

 fully into action. 



This property of points is well illustrated by their power of fa- 

 cilitating vaporization, or the release of a gas from solution. 

 When a point is placed in a fluid in the act of vaporizing, or 

 giving off gas which had been dissolved in it, by virtue of its 

 undisturbed attraction for the molecules of vapour which are 

 being formed, it soonest collects these molecules into bubbles; 

 and these collections of gas or vapour, therefore, at this point 

 first attain sufficient magnitude to overcome the resistance of the 

 surrounding medium, and ebullition starts from the point f. 



* It is interesting to observe that, of the gases employed in the researches 

 of Messrs. Jamin and Bertrand, hydrogen was the least absorbed ; thus, in 

 a vessel containing pounded glass and a free space of 590 cubic centims., 

 595 cubic centims. of hydrogen were taken in, 602 cubic centims. of air, 

 and 645 cubic centims. of CO 2 . These facts are noticeable in relation to 

 Graham's observation (Phil. Mag. vol. xxvi. p. 422) that hydrogen can 

 diffuse through graphite 3*8 times as rapidly as air, and Dumas's state- 

 ment that hydrogen will pass through the pores of a heated iron tube into 

 nitrogen. 



t It is important to notice, however, that metals with rough surfaces 

 Phil. Mag. S. 4. Vol. 33. No. 224. May 1867. 2 B 



