DIFFUSION OF GASES. GO 



air. If the pressure upon the air and gas were made equal,, then 

 the gas would be compressed into less bulk than the air, and 

 deviate from the law of Mariotte. Despretz has more lately 

 observed an equally conspicuous deviation from this law under 

 increasing pressures, in several other gases, particularly sul- 

 phuretted hydrogen, cyanogen, and ammonia, which are all 

 easily liquefied. There is no reason, however, to suppose that 

 any partial liquefaction of the gases occurs under the pres- 

 sure applied to them in such experiments. They remain 

 entirely gaseous, and their superior compressibility, must be 

 referred to a law of their constitution. It is the phenomenon 

 beginning to show itself in a gas under moderate pressure, 

 which was observed in all its excess by Cagnard de la Tour, in 

 the vapours confined by him under great pressure, (page 53.) 



Those gases, which exhibit this deviation, must occupy less 

 bulk than they ought to do under the pressure of the atmosphere 

 itself; which may be the reason why the liquefiable gases are 

 generally found by experiment to be specifically heavier than 

 they ought by theory to be. 



Such are the most remarkable features which gases exhibit 

 in relation to pressure and temperature. These properties are 

 independent of the specific weights of the gases, which are very 

 different in the various members of the class, and they are but 

 little connected with the nature of the particular substance or 

 material which exists in the gaseous form. But when different 



O 



gases are presented to each other, a new property of the gaseous 

 state is developed, namely the forcible disposition of different 

 gases to intermix, or to diffuse themselves through each other. 

 This is a property which interferes in a great variety of pheno- 

 mena, and is no less characteristic of the gaseous state than any 

 we have considered. It may be treated of under the head, 

 1 of the diffusion of gases through each other, and 2 of the 

 diffusion of vapours into gases, by which is meant, the ascent 

 of vapours from volatile bodies into air and other gases, of 

 which the spontaneous evaporation of water into the air is an 

 instance. 



Diffusion of gases. When a light and heavy gas are once 

 mixed together, they do not exhibit any tendency to separate 

 again, on standing at rest for some time, differing in this 

 respect from mixed liquids, many of which speedily sepa- 

 rate, and arrange themselves according to their densities, 



