344 EXPERIMENTS ON DIFFUSION IN RELATION TO 



of viscosity is obtained. Thus the numbers obtained by myself in 18G5 



are nearly double of those calculated by Prof. Stokes from the experiments 

 of Boily on pendulums, but not much more than half those deduced by 0. E. 

 Meyer from his own experiments. The other kind of diffusion is that of the 

 energy of agitation of the molecules. This is called the conduction of heat 

 The experimental investigation of this subject is confessedly so difficult, that 

 it is only recently that Prof. Stefan of Vienna*, by means of a very ingenious 

 method, has obtained the first experimental determination of the conductivity 

 of air. This result is, as he says, in striking agreement with the kinetic 

 theory of gases. 



The experiments on the interdiffusion of gases, as conducted by Prof. 

 Loachmidt and his pupils, appear to be far more independent of disturbing 

 causes than any experiments on viscosity or conductivity. The mterdifiuamg 

 gases are left to themselves in a vertical cylindrical vessel, the heavier gas 

 being underneath. No disturbing effect due to currents seems to exist, and the 

 results of different experiments with the same pair of gases appear to be 

 consistent with each other. 



They prove conclusively that the coefficient of diffusion varies inversely as 

 the pressure, a result in accordance with the kinetic theory, whatever hypo- 

 thesis we adopt as to the nature of the mutual action of the molecules during 

 their encounters. 



They also shew that the coefficient of diffusion increases as the temperature 

 rises, but the range of temperature in the experiments appears to be too small 

 to enable us to decide whether it varies as 2", as it should be according to the 

 theory of a force inversely as the fifth power of the distance adopted in my 

 paper in the Phil. Trans. 1866, or as jT* as it should do according to the 

 theory of elastic spherical molecules, which was the hypothesis originally developed 

 by Clausius, by myself in the Phil. Mag. 1860, and by O. E. Meyer. 



In comparing the coefficients of diffusion of different pairs of gases, Prof. 

 Loschmidt has made use of a formula according to which the coefficient of 

 diffusion should vary inversely as the geometric mean of the atomic weiglits 

 of the two gases. I am unable to see any ground for this hypothesis in the 

 kinetic theory, which in fact leads to a different result, involving the diameters 

 of the molecules, as well as their masses. The numerical results obtained l>y 



* Sitzb. d. k. Akad. Feb. 22, 1872. 



