

in liberating Gas from Solution. 305 



the side^below the surface, introduces air and not oil — or that a 

 clean glass rod on which no bubbles appear, taken out and 

 drawn with friction through the hand, acquires an air coating, 

 which causes it to be covered with bubbles when returned to 

 the soda-water. A man must be strangely fascinated with his 

 own hypothesis to maintain such a proposition; and yet M. 

 Gernez, after a lapse of eight years, attempts to explain my 

 results (which have been confirmed by Professor Schrotter *) 

 by seriously maintaining that the effect of washing with caustic 

 potash solution, boiling distilled water, or alcohol is not to 

 make the solid surfaces chemically clean, but to deprive them 

 of the film of air which clings to them. I quote his language : 

 — " Si Pon a soin de dissoudre, par des lavages successifs k la 

 potasse, a Peau distillee bouillante et a Palcool, la couche super- 

 ficielle des vases de verre en certains points de laquelle se trou- 

 verait retenue une petite quantite d'air, on constate qu'il ne se 

 forme plus une seule bulle gazeuse sur la paroi baignee par le 

 liquide, pas plus qu'a Pinterieur de la solution sursaturee entre 

 des limites de temperature et de pression tres-etendues " (p. 44). 



As to the action of porous bodies in liberating gas from solu- 

 tion, I have long since endeavoured to show that they act by 

 reason of that strong adhesive force of which the absorptive 

 power of charcoal for various gases affords striking examples. 

 Charcoal, or other porous body, in a boiling liquid (which I have 

 defined as a supersaturated solution of its own vapour f) power- 

 fully absorbs the vapour of that liquid, but cannot retain it so 

 long as the liquid is maintained at or near its boiling-point; 

 hence it is constantly pouring forth streams of vapour, and by 

 its presence so far facilitates the escape of vapour, that in dis- 

 tillations from 20 to 30 percent, more liquid is condensed in 

 consequence of the presence of a few bits of charcoal, while at 

 the same time soubresauts are prevented. 



It appears to me that porous bodies liberate gas from gaseous 

 supersaturated solutions by virtue of that same force of adhesion 

 by which gases become imprisoned in charcoal &c. A piece of 

 cocoanut-shell charcoal, for example, in soda-water, condenses 

 a certain amount of carbonic acid within its pores ; and the ad- 

 hesive force continues active even though a further supply of 

 gas can find no entrance. In seeking to enter, it becomes dis- 

 engaged from the water, and rises in a stream apparently from 

 the surface of the charcoal. Under diminished pressure the 

 gas escapes in large quantities from the charcoal itself, while 

 the adhesive force is restored with the pressure. 



* Pogg. Ann. vol. cxxxvii. 



t" Proceedings of the Royal Society, January 21, 186.9. 



