Prof. Graham on the Law of the Diffusion of Gases. ISO 



It is evident from this, that the air does not diffuse out 

 against so strong a pressure and the inward current of hy- 

 drogen. 



When this jar, of which the capacity was 65 cubic inches, 

 was used as a diffusion-instrument, and filled over water with 

 hydrogen, one fourth of the hydrogen which it contained 

 escaped by diffusion into air in the first hour. Now, we find 

 by the table, (p. 188.), that hydrogen penetrates the plug with 

 greater velocity when passing into a vacuum or into the ex- 

 hausted receiver. The exhausted receiver was filled one 

 fourth in about fifteen minutes ; hence a certain quantity of 

 hydrogen passed through the same porous plug, by the pres- 

 sure of the atmosphere, into a vacuum in fifteen minutes ; by 

 spontaneous diffusion into air in sixty minutes ; or the velo- 

 city of diffusion was one fourth the velocity of mechanical pres- 

 sure. 



This was a dense and excellent plug ; and in others of a looser 

 texture, the velocity of diffusion was much less than a fourth. 



Dried bladder answers for showing the diffusion of hydro- 

 gen when stretched over the open end of the tube receiver. 

 The diffusion, however, through a single thickness of bladder, 

 is effected at least twenty times more slowly than through a 

 thickness of one inch of stucco. While, on the other hand, 

 either air or hydrogen, under mechanical pressure, passes 

 more readily through bladder than a great thickness of stucco. 

 Gold beaters' skin is even more permeable by gases under a 

 slight pressure than bladder, and less suitable for diffusion. 



The superior aptitude of stucco for exhibiting the unequal 

 diffusion of gases of different densities, seems to depend upon 

 its pores being excessively numerous, but exceedingly minute, 

 making in the aggregate a considerable channel. In the blad- 

 der, or goldbeaters' skin, the pores I suppose to be few in 

 number but wide, making, however, when added together, but 

 a small channel. Air passes through them but little impeded 

 by friction. 



Dry and sound cork answers exceedingly well as a substi- 

 tute for the stucco-plug. The diffusion takes place slowly, 

 but is not apt to be deranged by a slight mechanical pressure. 

 So do thin laminae of many granular minerals, such as the 

 flexible magnesian limestone, &c. ; charcoal also, and woods, 

 if not too porous, may be applied to the purpose. 



It might occur, in explanation of our experiments with the 

 diffusion-instrument, to take Mr. Dalton's hypothesis, and 

 suppose, in the case of hydrogen, the external air to be a va- 

 cuum to the hydrogen, and the hydrogen a vacuum to the 

 air, and that the inequality of the diffusion depends upon the 



