[From Nature, Vol. vin.] 



LIX. On Loschmidfs Experiments on Diffusion in relation to the Kinetic 



Theory of Gases. 



THE kinetic theory asserts that a gas consists of separate molecules, each 

 moving with a velocity amounting, in the case of hydrogen, to 1,800 metres 

 per second. This velocity, however, by no means determines the rate at which 

 a group of molecules set at liberty in one part of a vessel full of the gas 

 will make their way into other parts. In spite of the great velocity of the 

 molecules, the direction of their course is so often altered and reversed by 

 collision with other molecules, that the process of diffusion is comparatively a 

 slow one. 



The first experiments from which a rough estimate of the rate of diffusion 

 of one gas through another can be deduced are those of Graham "". Professor 

 Loschmidt, of Vienna, has recently f made a series of most valuable and accurate 

 experiments on the interdiffusion of gases in a vertical tube, from which he 

 has deduced the coefficient of diffusion of ten pairs of gases. These results I 

 consider to be the most valuable hitherto obtained as data for the construction 

 of a molecular theory of gases. 



There are two other kinds of diffusion capable of experimental investigation, 

 and from which the same data may be derived, but in both cases the experi- 

 mental methods are exposed to much greater risk of error than in the case 

 of diffusion. The first of these is the diffusion of momentum, or the lateral 

 communication of sensible motion from one stratum of a gas to another. This 

 is the explanation, on the kinetic theory, of the viscosity or internal friction 

 of gases. The investigation of the viscosity of gases requires experiments of 

 great delicacy, and involving very considerable corrections before the true 



* Brande's Journal for 1829, pt. ii. p. 74, "On the Mobility of Gases," Phil. Train. 1863. 

 t Sitzb. d. L Akad. d. Wissench. 10 Marz. 1870. 



