at Very Loiv Pressures. 15 



Warburg and Ihrnori (Wied. Awn. xxvii., xxxi.) and others 

 show that there is a chemical reason for part of the strong 

 attraction of glass for water, since fresh glass has an alkaline 

 film on its surface which can be dissolved off by water — a fact 

 which explains the power that fresh glass surfaces have of con- 

 densing moisture from air which is far from being saturated. 



• • • 



This water is easily got rid of, but there is water attached 



more firmly which can be driven off a glass surface only by 

 heating at very low pressure. It seems to me that we have 

 to do here with a sort of solution of water in the solid glass, 

 and not with a genuine surface condensation ; but the behaviour 

 of water on a glass surface has been generally held to be 

 only an extreme case of the behaviour of any gas ; if, how- 

 ever, there is the great distinction which has just been sug- 

 gested, there is no force in the inference from water. 

 Carbon dioxide also comes freely from a glass surface 

 sometimes, but Krause (Wied. Ann. xxxvi.) seems to have 

 shown that the presence of water- vapour is necessary. Thus 

 there seems to be no genuine evidence that gases condense in 

 amounts hitherto measurable on ordinary solid surfaces. Of 

 course, with an easily liquefiable gas below its critical tempe- 

 rature, we can imagine that if the attraction of the solid for 

 the gas is much greater than the attraction of the gas for 

 itself, a thin layer of gas liquefied on the surface would 

 evaporate with greater difficulty than if it were on the surface 

 of a large mass of itself ; but from what we now know of 

 molecular force we should expect such an effect merely to 

 modify somewhat for such a layer the ordinary laws of 

 evaporation, but not to alter them entirely, as the supposition 

 of a retention of a layer of liquid at pressures far below that 

 of saturation would necessitate ; the retention of an appre- 

 ciable amount of volatile substance, as of water, on glass 

 brings us back to causes more of a chemical nature, as we have 

 just seen. 



Thus, with the lav/ of molecular force as that of the inverse 

 fourth power, we are led to the conclusion that condensation 

 of gases on ordinary solid surfaces at pressures far removed 

 from those of liquefaction does not occur to an extent mea- 

 surable by methods hitherto applied, and that apparent 

 departure from Boy le 5 s law on account of surface condensation 

 is too small to have been hitherto detected, and does not become 

 relatively any larger in rare than in dense gases. The causes 

 for any apparent breakdown of Boyle's law in rarefied gases 

 are therefore to be sought for elsewhere than in surface con- 

 densation caused by molecular force if the law of that force is 

 that of the inverse fourth power. 



