354? Prof. Graham on the Law of the Diffusion of Gases* 



Now, the gases were found by analysis to be uniformly dif- 

 fused through both bulbs; so that nitrogen and carbonic oxide 

 are of the same density, or at least do not differ more than 

 j^dth part, which was the limit of observation in the case of 

 these experiments. It appears, also, that inequality of density 

 is not an essential requisite in diffusion. 



I had occasion to remark, more than once, a singular acci- 

 dent to the stucco plugs. After being disused for some days 

 or weeks, and left in the interval exposed to the air, which 

 might be either dry or damp at the time, the plugs occasion- 

 ally, on a new trial, did not permit diffusion to take place 

 through their pores, at least immediately. Hydrogen, how- 

 ever, always opened a passage in the course of two or three 

 minutes, and then the diffusion proceeded as rapidly as ever. 

 Carburetted hydrogen, and the other gases, often required a 

 longer period. A slight heat restored the action of the plug. 

 The obstruction could not be attributed 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 ex- 

 pressing their diffusion-volumes; — nothing that could be con- 

 sidered 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 prin- 

 ciples of diffusion. The powerful disposition of the particles 

 of different gaseous bodies to exchange positions, may as ef- 

 fectually induce the first separation of vapour from the surface 

 of the liquid, as a vacuum would do. Once elevated, the va- 

 pour will be propagated to any distance, by exchanging posi- 

 tions 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 distance. 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 

 diffusion, in explanation of the mechanism of respiration. The 

 cavity 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 indeterminate extent, till at last the tubes cease to be of 

 sensible magnitude, but are believed to terminate in shut sacs. 

 The capacity of the whole cavity cannot easily be determined, 

 but we may estimate it at 300 cubic inches. In a natural ex- 



