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



the quantity of return-air is uniformly greater than it should 

 be. Thus 3*65 and 369 were the diffusion-volumes of hy- 

 drogen deduced from an experiment, in the one case with a 

 plug which had been dried at 100°, and subsequently exposed 

 for several hours to the air, and in the other case, with a plug 

 merely dried in air, temperature 68°. The obvious cause of 

 this is, that the air is dried in passing through the plug, and 

 is subsequently expanded while in the diffusion-instrument by 

 the ascent of vapour into it. Hence, the first time a diffusion- 

 bulb is tried, it generallv gives the diffusion-volume of hydro- 

 gen below the truth. 



On the other hand, I apprehend, that when the pores of 

 the stucco are saturated with hygrometric moisture, which, 

 from the circumstances of the experiments, must be almost al- 

 ways the case, the hydrogen, in making its way through the 

 plug, actually avails itself to a small extent of this moisture, 

 inducing it to vaporize, and exchanging places with it instead 

 of air. Hydrogen which escapes in this way will not be re- 

 presented by return-air, the quantity of which is thus dimi- 

 nished. This process, however, is extremely intricate, and 

 has not yet been fully investigated. Its effect is insensible in 

 the case of the other gases, of which the diffusion- volumes 

 approach more closely to that of air. 



The more dense and compact the plaster-plug, the more 

 correct appear to be its general indications. On this account 

 I compress the plug, while moist, before it sets. When the 

 plug is of a loose structure, and probably contains sensible 

 vacuities in its substance, diffusion goes on with increased ra- 

 pidity; but I have observed, that the proportion of return-air 

 is notably diminished in the case of the diffusion of hydrogen. 

 Thus, in a set of experiments with a diffusion-bulb, having a 

 plug of this description, and little more than one-tenth of an 

 inch in thickness, I obtained, as the diffusion- volume of hy- 

 drogen, 4*05, 4*04, and 400. This plug had been somewhat 

 thicker at one time, and then gave 3*93 as the diffusion-volume 

 of hydrogen. These experiments exhibit an extreme case of 

 this deviation. It appears to depend upon some physical 

 property of hydrogen gas which is peculiar to it. To obtain 

 light upon this subject, I was led to investigate the rate at 

 which air, hydrogen, and the other gases flow through the 

 stucco-plug into a vacuum, under the influence of mechanical 

 pressure. 



A small bell jar, with an opening at top, was used, which 

 opening was closed with a plug of Paris piaster tif half-an-inch 

 in thickness, over which a brass cap and stopcock were fitted 

 and cemented. This receiver was placed on the plate of an 



