PROFESSOR THOMAS GRAHAM's SCIENTIFIC WORK. 201 



" The pores of artificial graphite appear to be really so miuute that a 

 gas in mass cauuot 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 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 condition of matter. 



" According to the physical hypothesis now generally received, a gas 

 is represented as consisting of solid and perfectly elastic spherical par- 

 ticles or atoms, which move in all directions, and are animated with dif- 

 ferent degrees of velocity in different gases. Confined in a vessel, the 

 moving particles are constantly impinging against its sides and occasion- 

 ally against each other, and this contact takes place without any loss of 

 motion, owing to the perfect elasticity of the particles. If the contain- 

 ing vessel be porous, like a diffusiometer, then gas is projected through 

 the open channels, hj the atomic motion described, and escapes. Simul- 

 taneously the external air is carried inward in the same manner, and 

 takes the place of the gas which leaves the vessel. To this atomic or 

 molecular movement is due the elastic force, with the power to resist 

 compression, possessed by gases. The molecular movement is accelera- 

 ted 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, or is 

 in contact with both sides of our porous plate, the movement is sustained 

 without abatement — molecules continuing to enter and leave the vessel 

 in equal number, 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, as 

 nitrogen and carbonic oxide do, an interchange of molecules also takes 

 place without any change in volume. With gases opposed of unequal 

 density and molecular velocitj^, the permeation ceases of course to be 

 equal in both directions." 



One set of novel experiments recorded in the later i^aper, from which 

 the above remarks are extracted, had reference to the diffusion of single 

 gases through porous septa, into a vacuous or partially vacuous space. 

 The diffusion-tube was substantially the sameas that formerly employed, 

 except in the circumstance of its being closed by a jilate of compressed 

 graphite instead of by stucco, and in the further circumstance of the 

 tube itself being in some cases so far lengthened and otherwise modified 

 as to admit of the production within it of a barometric vacuum of com- 

 paratively large dimensions. The mode of experimenting was as fol- 

 lows : The short tubes, when employed, were filled with mercury, and 

 inverted in a mercurial trough. Then, by means of a very simple 

 arrangement, the gas under examination was allowed to sweep over the 

 surface of, and diffuse through, the graphite plate, so as to depress the 



