322 Mr. T. S. Thomson's Observations on Mr. Graham's 



in consequence of the great diffusive velocity of hydrogen, the 

 external level is maintained by fresh additions of water, so as 

 to avoid the mechanical agency of an increasing atmospheric 

 pressure. At the conclusion of the experiment, when the 

 hydrogen has entirely escaped, and the level become station- 

 ary, the quantity of replacing atmospheric air is carefully 

 measured, and compared with the known volume of hydro- 

 gen originally introduced into the instrument. The fraction 



original vol. of hydrogen , ..„, . , „. , 



— - — j — : if- — = the diffusion volume or hydrogen 



replacing vol. or air J ° 



referred to that of air as unity. The mean of five experiments 

 gave 3*848 as the diffusion volume of hydrogen, in accordance 

 with the law announced. For the specific gravity of hydro- 

 gen being 0-0694, the square root of which is 0-2635, we have 

 the proportion 0-2635 : 1 : : 1 : 3-7947, the diffusion volume of 

 hydrogen, a number approaching very closely to that indi- 

 cated by experiment. Carbonic acid gas, chlorine, sulphu- 

 rous acid gas, protoxide of nitrogen, and other gases, operated 

 upon in the same manner, present similar results, all tending 

 to show that their respective diffusion volumes are in the in- 

 verse ratio of the square roots of their densities. 



With a view to explain some apparent inconsistencies in 

 the results of experiments made under varied circumstances, 

 Mr. Graham investigates the rate at which different gases, 

 under the influence of mechanical pressure, flow through small 

 apertures into a vacuum. Operating as in the former case, 

 with a plaster of Paris plug as the porous medium, different 

 gases were allowed to penetrate through it into an exhausted 

 receiver ; the comparative velocities of entrance were inferred 

 from the indications of a mercurial barometer gauge attached 

 to the apparatus. The same volume of different gases under 

 the same pressure entered in the times expressed in the fol- 

 lowing Table, beginning at a pressure of 29 inches, and ter- 

 minating in a pressure of 27 inches of mercury. 



Air, dry 10 m s 



Air, saturated with moisture at 60°... 10 



Carbonic acid 10 



Nitrogen 10 



Carbonic oxide 9 30 



Olefiant gas 7 50 



Coal gas 7 



Hydrogen 4 



The velocity of the same gas under different pressures was 

 found to vary with the pressure, but not in direct proportion 

 to it. Under twofold pressure the velocity was not quite 



