the Diffusion of Gases. 237 



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 cor- 

 rect 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 rapidity ; but I 

 have observed, that the proportion of return-air is notably dimi- 

 nished in the case of the diffusion of hydrogen. Thus, in a set 

 of experiments with a diffusion-bulb, having a plug of this de- 

 scription, and little more than one-tenth of an inch in thickness, 

 I obtained, as the diffusion- volume of hydrogen, 4.05, 4.04, and 

 4.00. This plug had been somewhat thicker at one time, and 

 then gave 3.93 as the diffusion-volume of hydrogen. These ex- 

 periments 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 plaster of 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 air-pump 

 in perfect order, and exhausted. When the stopcock of the re- 

 ceiver was closed, nothing entered the exhausted receiver ; but 

 on opening it, either air entered, forcing its way through the 

 pores of the stucco, or any gas which might be conducted to it, 

 by means of a flexible tube from a proper magazine. 



The time was noted in which the mercury of the gauge-ba- 

 rometer, in communication with the receiver, fell two inches, al- 

 ways setting out with gas of the tension of one inch mercury in 

 VOL. xn. PART i. H h 



