14 Mr. W. Sutherland on Boyle's Law 



large this surface may pass into the pore and clothe all the 

 interspaces ; whereas if the pores are fine it may show only a 

 slight depression opposite a surface-pore and have no existence 

 in the inner interspaces. Thus it would not be difficult to find 

 the surface of constant pressure which is just prevented from 

 entering an average pore, and so determining p c as a function 

 of pj., size of pore, and molecular force, and so to find the 

 theory of Kayser's experiments on the compression of gases 

 in contact with powders (Wied. Ann. xiv.). For present 

 purposes it is enough to say that by varying the size of the 

 interspaces and the nature of the powder or fibre he was able 

 to demonstrate distinct variation of the mass of gas adsorbed, 

 namely p c B ei with NH 3 , C0 2 , and 80 2 . With boxwood char- 

 coal (Wied. Ann. xii.) Kayser, in confirmation of previous 

 investigators, such as Chappuis and Joulin, found a measurable 

 adsorption of both air and hydrogen ; for instance, 1 c.c. of 

 charcoal, of which about two-thirds is interspace, adsorbs 

 about 3 c.c. of air at a pressure of one atmo and at 0° C. and 

 about 1*5 c.c. of hydrogen at 14° C, but at 50° scarcely a trace 

 of air is adsorbed— a fact which shows that the circumstances 

 in the case of charcoal are quite special, and though interesting 

 can throw no light on the present inquiry as to the general 

 action of solid surfaces on gases. 



If we return to equation (31) in the form 



M = P B + M/3/k-$zJ Ps -p)/2, . . . . (33) 



we see that in general the surface-term must be smaller than 

 Mj3/k — that is to say, than a constant small fraction of the 

 mass for a given gas — and therefore that however much we 

 may rarefy a gas the relative importance of the surface-term 

 can never exceed a certain small limiting amount. This is in 

 direct contradiction to the prevailing impression that surface 

 effect becomes of increasing importance with the rarefaction. 

 We have therefore to consider the origin of the general 

 impression, which is twofold, namely, first, the theoretical 

 belief that the mass of a gas ought to be given by an equation 

 of the form 



M = P B + S(p-p), 



where, in contrast to our equation (33), the surface-term has a 

 positive sign, and there is no obvious reason why the surface- 

 term ought not to become of increasing importance with 

 rarefaction because of p s diminishing more slowly than p ; and 

 secondly, the well-known experimental difficulties of getting 

 rid of the last traces of volatile matter from the walls of glass 

 vessels in preparing vacuum bulbs ; but the experiments of 



