the Diffusion of Gases. 241 



fected at least twenty times more slowly than through a thick- 

 ness 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. Goldbeaters 

 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, mak- 

 ing in the aggregate a considerable channel. In the bladder, 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 substitute 

 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 sup- 

 pose, in the case of hydrogen, the external air to be a vacuum to 

 the hydrogen, arid the hydrogen a vacuum to the air, and that 

 the inequality of the diffusion depends upon the hydrogen being 

 least resisted in passing through the plug. The experiments on 

 the permeability of the stucco by gases under pressure, above 

 detailed, were projected with a view to settle this point among 

 others ; and they are evidently incompatible with such an appli- 

 cation of the theory, for hydrogen passes 2.4 times more swiftly, 

 and not 3.8 times, as in the diffusion experiments. Carbonic acid, 

 too, permeates the plug, under pressure, as rapidly as air does, or 

 even somewhat more rapidly, for our results inclined to this side 

 rather than to the other ; whereas carbonic acid diffuses through 



