Prof*. Graham on the Law of the Diffusion of Gases. 181 



In a diffusion-tube standing over water, temperature 65°, 

 88 volumes hydrogen were replaced by 26 air; 84 hydrogen 

 by 25 air; and in another tube, 130 hydrogen by 38 air. 

 The quantity of return-air is here related to the hydrogen 

 diffused, as 1 to 3*38, 3*36, and 3*42, numbers which approach 

 to, but fall short of, the theoretical diffusion-volume of hydro- 

 gen, namely, 3*79. But the hydrogen in these experiments 

 was saturated with vapour at 65° 9 which would make its den- 

 sity 0*0809, and reduce its diffusion-volume to 3*5161 ; while 

 the air without, being comparatively dry, would be somewhat 

 expanded after it entered the diffusion-tube, by the ascent of 

 vapour into it. This would occasion the quantity of return- 

 air to appear greater than it should be ; but it is difficult to 

 find elements for a proper correction, as not only the quantity 

 of vapour in the atmosphere must be taken into account, but 

 also the hygrometric state of the plug itself. The increased 

 return-air, however, evidently lowers the diffusion-volume of 

 the hydrogen gas. 



With the view of increasing the capacity of the instrument, 

 and the number of its divisions, and of obviating the interfe- 

 rence of vapour, the mode of performing the 

 experiment was varied. On a tube, four 

 tenths of an inch in diameter, a bulb of two 

 inches in diameter was blown, as in figures 

 A and B. The tube above and below the 

 bulb, in the case of A, was graduated into 

 two-hundredths of a cubic inch. The upper 

 end of the tube was closed by stucco, as in 

 the case of the simple diffusion-tube. The 

 general mode of proceeding will be best con- 

 ceived from the recital of the details of a par- 

 ticular experiment. 



The diffusion-instrument employed in the following experi- 

 ment contained 855 measures, and was of the form A. The 

 stucco plug was unusually large, being 0*6 inch in length, 

 which occasioned the diffusion to be slow. At the commence- 

 ment of the experiment the thermometer stood at 68°, and the 

 barometer 29*73 inches. The bulb being sunk in water with 

 the air-syphon in it, the whole air was withdrawn, with the 

 exception of 12 measures, and the instrument filled up with 

 newly made hydrogen gas. So that at the outset we had in 

 the instrument, 



Air with its vapour 12* 



Hydrogen 823*83 



Vapour (accompanying the hydrogen at 68°) 19*17 



855*00 



