410 Mi*. T. Graham on the Molecular Mobility of Gases. 



circular disc of this graphite, which Fig. 1. Fig. 



is like a wafer in thickness but pos- 

 sesses considerable tenacity, is at- 

 tached by resinous cement to one 

 end of the glass tube above described, 

 so as to close it and form a diffusio- 

 meter (fig. 1). The tube is filled 

 with hydrogen gas over a mercurial 

 trough, the porosity of the graphite 

 plate being counteracted for the time 

 by covering it tightly with a thin 

 sheetof guttapercha (fig. 2). On after- 

 wards removing the latter, gaseous 

 diffusion immediately takes place 

 through the pores of the graphite. 

 The whole hydrogen will leave the 

 tube in forty minutes or an hour, and 

 is replaced by a much smaller pro- 

 portion of atmospheric air (about 

 one-fourth), as is to be expected 

 from the law of the diffusion of gases. 

 During the process, the mercury will 

 rise in the tube, if allowed, forming 

 a column of several inches in height 

 — a fact which illustrates strikingly 

 the intensity of the force with which the interpenetration of differ- 

 ent gases is effected. Native graphite is of a lamellar structure, 

 and appears to have little or no porosity. It cannot be substi- 

 tuted for the artificial graphite as a diffusion-septum. Unglazed 

 earthenware comes next in value to graphite for that purpose. 



The pores of artificial graphite appear to be really so minute, 

 that a gas in mass cannot penetrate the plate at all. It seems 

 that molecules only can pass; and they may be supposed to 

 pass wholly unimpeded by friction, for the smallest pores that 

 can be imagined to exist in the graphite must be tunnels in 

 magnitude to the ultimate atoms of a gaseous body. The sole 

 motive agency appears to be that intestine movement of mole- 

 cules which is now generally recognized as an essential property 

 of the gaseous condition of matter. 



According to the physical hypothesis now generally received*, 

 a gas is represented as consisting of solid and perfectly elastic 

 spherical particles or atoms, which move in all directions, and are 



* D. Bernoulli, J. Herapath, Joule, Kronig, Clausius, Clerk Maxwell, 

 and Cazin. The merit of reviving this hypothesis in recent times and first 

 applying it to the facts of gaseous diffusion, is fairly due to Mr. Herapath. 

 See 'Mathematical Physics,' in two volumes, by John Herapath, Esq. (1847). 



