ABSORPTION OF GASES BY SOLIDS. 87 



as they do on stirring the liquid with a spoon or rough object, the 

 effect being heightened by the warming of the liquid in the mouth, 

 which promotes the disengagement of gas bubbles. 



CHAPTER VII. 



SOLUTION OF GASES IN SOLID SOLVENTS AND SEPARATION OF 

 GASES FROM SOLID SOLUTIONS, NO CHEMICAL ACTION 

 TAKING PLACE. 



Just as liquids will absorb or dissolve gases, forming solutions 

 thereof, so can solids to some extent absorb or occlude gases; and just 

 as the solubility of a gas in a liquid depends on the nature of the gas 

 and on that of the liquid, and also on the temperature and pressure 

 and such like considerations, so also is the quantity of gas absorbed 

 by a solid influenced by the same circumstances. In most cases, 

 in order to extract the gas dissolved by or occluded in a solid, it is 

 requisite to heat it highly in a vessel whilst the pressure is largely 

 diminished by exhausting by some form of air-pump ; but in some 

 instances the dissolved gas is easily expelled by heat alone. 



Thus, when metals of various kinds are heated, they frequently 

 part with gases which they have previously absorbed, either during 

 the process of their manufacture or subsequently. Sometimes a 

 metal will absorb a large quantity of one particular kind of gas, but 

 only comparatively small amounts of other kinds ; thus iron pos- 

 sesses the property of freely absorbing the gas termed carbon oxide 

 or carbon monoxide (not the same thing as the " carbon dioxide " 

 or carbonic acid gas used for aerating water, &c.), and the somewhat 

 rare and costly metal palladium can absorb several hundred times 

 its bulk of hydrogen gas. On heating in an exhausted vessel the 

 absorbed gas is released, and can be pumped out with facility. 

 India-rubber possesses the property of absorbing oxygen pretty 

 freely, and on this property is based a method of separating oxygen 

 from the air. Silver possesses the peculiar property of absorbing 

 oxygen when it is very hot indeed, and considerably above its 

 melting-point ; whereas on cooling somewhat most of the dissolved 

 gas is given off again, thus causing the molten metal to "spit" or 

 effervesce, somewhat after the fashion of soda-water, but much less 

 vigorously. 



Expt. 81. Absorption of Hydrogen by Palladium. One of the 



