iiz ATMOLYSIS. PAETI. 



through, a minute opening in a very thin plate into a 

 vacuum ; transpiration, or the passage of gases through 

 capillary tubes into vacuo; and lastly atmdiysis, which 

 is the partial separation of a mixture of gases and 

 vapours of different degrees of diffusibility by permit- 

 ting them to diffuse themselves through a porous plate 

 into a vacuum : a new kind of analysis, which possesses 

 a practical character of extensive application. 



The diffusing instrument employed by Mr. Graham 

 was a cylindrical glass tube about an inch in diameter, 

 ten inches long, with one end closed by a very thin 

 porous disc of compressed artificial graphite fixed by a 

 resinous cement. While the tube was being filled with 

 hydrogen gas over a trough of mercury, the escape of 

 the gas was prevented by covering the graphite very 

 carefully with a thin sheet of gutta percha. As soon as 

 the gutta percha was removed, the reciprocal diffusion 

 of the gases began, and in from forty to sixty minutes 

 the whole of the hydrogen had escaped from the tube, 

 and a quantity of atmospheric air amounting to about 

 one fourth of the volume of hydrogen had entered the 

 tube and taken its place, according to the ordinary law 

 of the diffusion of gases. During this time the mercury 

 rises in the tube so as to form a column several inches 

 high, a fact which is a striking demonstration of the 

 intensity of the force with which the reciprocal pene- 

 tration of different gases effected. 



Natural plumbago or graphite has little or no porosity 

 and cannot be used in these experiments, but the pores 

 of artificial graphite of which pencils are made, appear 

 to be so minute that only isolated molecules of gas are 

 able to pass, without however being at all impeded by 

 friction ; for the smallest pores that we can suppose to 

 exist in the graphite must be real tunnels compared 

 with the minuteness of the ultimate atoms or molecules 

 of a gaseous body. The cause of motion appears to 



