ADHESION OF GASES TO SOLIDS. 233 



the accuracy of its indications; in well-made instruments this air 

 is got rid off by boiling the mercury in the tube ; the adherent air 

 is partly dislodged by the heat alone, and partly becomes replaced 

 by mercury vapour, and is thus driven out whilst the tube is 

 hot. 



In making electrical "vacuum tubes," or sealed glass tubes from 

 which all air or other gas contained is pumped out to the highest 

 possible extent, a similar precaution has to be taken, otherwise 

 the film of air adherent to the glass gradually becomes dislodged, 

 and more or less spoils the vacuum. 



Gases thus attracted to the surface of solids bear to gases truly 

 dissolved or occluded by solids much the same relationship that 

 a film of water adhering to the outside of a gold or silver coin, but 

 not penetrating into its interior, bears to mercury brought in con- 

 tact with the metal; after a time (Expt. 69) the mercury creeps 

 into the interior of the solid gold or silver; and, if present in 

 sufficient quantity, manifests its presence by rendering the whole 

 brittle ; and in the case of gold by visibly whitening its tint, so 

 that the fractured surface is much paler in shade than it would be 

 were no mercury present. 



As the adhesion of gases to the surface of solid bodies is only a 

 superficial action, the adherent film not penetrating into the 

 interior (just as a compact solid dipped in water is only wetted 

 externally), it would naturally be expected that if the amount of 

 surface is increased relatively to the weight of the solid by dividing 

 it up into small pieces or powdering it, a given weight of solid 

 would take up more gas the more finely it is divided, just as more 

 water would be taken up by a given weight of fine sand than by 

 the same weight of a compact block of stone. Not only is this 

 the case, but further, the gases thus made to adhere in compara- 

 tively large quantity to the surfaces of finely divided solids exhibit 

 the same peculiarity as gases dissolved in or condensed by solids 

 in their interior, i.e., their chemical activity is greatly increased 

 (Expt. 83); and, in consequence, gases thus condensed upon the 

 surface of finely powdered solids, or of solid matters obtained by 

 special devices in highly porous masses like charcoal, often possess 

 the power of setting up chemical changes which the same gases in 

 the ordinary free state (as they would be contained in a jar or 

 bottle) are quite incapable of bringing about. Certain metals, 

 such as platinum, not naturally prone to rusting or " oxidation," 

 can be obtained by particular chemical processes in a fine state of 

 division ; and when in this " spongy " condition are well adapted 

 for the illustration of phenomena of this kind. With certain 

 gases, however, a very fine state of division is not requisite, thin 



