168 ANNUAL OF SCIENTIFIC DISCOVERT. 



than an inch in diameter and about ten inches in length, was simply 

 closed at one end by a porous plate of plaster-of-paris, about one-third 

 of an inch in thickness and thus converted into a gas-receiver. A su- 

 perior material for the porous plate is now found in artificially com- 

 pressed graphite, of the quality used for making writing-pencils. A 

 circular dish of this graphite, reduced to the thickness of a wafer (one 

 half a millimetre) is attached, by resinous cement to one end of the 

 glass tube, above described, so as to close it and form a diffusiometer. 

 The tube is filled with hydrogen gas over a mercurial trough, the po- 

 rosity of the graphite plafce being counteracted for the time by cov- 

 ering it tightly with a thin sheet of gutta-percha. On afterwards re- 

 moving 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 propor- 

 tion 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 different gases is effected. The 

 native or mineral graphite is of a lamellar structure, and appears to 

 have little or no porosity. It cannot be substituted for the artificial 

 graphite as a diffusion septum. - Unglazed earthenware comes next 

 in value to graphite for this 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 to be molecules only which can pass ; and these 

 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 molecules which is 

 now generally recognized as an essential property of the gaseous con- 

 dition of matter. According to the physical hypothesis now generally 

 received, a gas is represented as consisting of solid and perfectly elas- 

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

 animated with different degrees of velocity in different gases. Con- 

 fined in a vessel, the moving particles are constantly impinging against 

 its sides, and occasionally against each other, and such collisions take 

 place without any loss of motion, owing to the perfect elasticity of the 

 particles. Now, if the containing vessel be porous, like a diffusiometer, 

 then gas is projected through the open channels by the atomic motion 

 described, and escapes. Simultaneously, the external air or gas, what- 

 ever it may be, is carried inwards in the same manner, and takes the 

 place of the gas which leaves the vessel. To the same atomic or mole- 

 cular movement is due the elastic force, with the power to resist com- 

 pression, possessed by gases. The molecular movement is accelerated 

 by heat and retarded by cold ; the tension of the gas being increased 

 in the first instance and diminished in the second. Even when the 

 same gas is present, both within and without the vessel, and is therefore 

 in contact with both sides of the porous plate, the movement is sus- 

 tained without abatement ; molecules continuing to enter and leave 

 in equal numbers, although nothing of the kind is indicated by change 

 of volume or otherwise. If the gases in communication be different, 

 but possess sensibly the same specific gravity and molecular velocity, 



