the Diffusion of Gases. 55 



stored the action of the plug. The obstruction could not be at- 

 tributed to moisture, nor to any thing but dust. 



It may be mentioned, that there was nothing peculiar in a 

 mixture of two gases, in the proportion of the numbers express- 

 ing their diffusion-volumes ; nothing that could be considered 

 an indication of mutual saturation. 



Evaporation, or the elevation of vapour from a liquid into 

 air, or any other gas, comes now to be explained on the principles 

 of diffusion. The powerful disposition of the particles of diffe- 

 rent gaseous bodies to exchange positions, may as effectually in- 

 duce the first separation of vapour from the surface of the liquid, 

 as a vacuum would do. Once elevated, the vapour will be pro- 

 pagated to any distance, by exchanging positions with a train of 

 particles of air, according to the law of diffusion. The length 

 to which this diffusion proceeds, in a confined portion of air, is 

 limited by a property of vapour, namely, that the particles of any 

 vapour condense when they approximate within a certain dis- 

 tance. Hence, the quantity of vapour which rises into air, has 

 the same limit as that which rises into a vacuum, and is the 

 same. 



I may be allowed to mention an application of the law of dif- 

 fusion, in explanation of the mechanism of respiration. The ca- 

 vity into which air enters during respiration, consists, first, of a 

 large tube, the windpipe ; secondly, of smaller tubes, into which 

 the windpipe diverges ; and, thirdly, of a series of still smaller 

 tubes, diverging from the last, themselves ramifying to an inde- 

 terminate extent, till at last the tubes cease to be of sensible mag- 

 nitude, but are believed to terminate in shut sacs. The capacity 

 of the whole cavity cannot easily be determined, but we may esti- 

 mate it at 300 cubic inches. In a natural expiration, about 20 

 cubic inches, or T Vth of the contents are thrown out, from the 

 application of a general pressure to the whole. But it is evident, 

 that these twenty cubic inches will be the twenty cubic inches 

 nearest the outlet, or the contents of the larger tubes. The con- 



