526 Mr. W. Sutherland on the Viscosity 



results, and if the experimental difficulties were not so great, 

 we should be led to the conclusion that the theory is inadequate ; 

 but the variation of the individual numbers from which these 

 means are derived is so great that we cannot yet accept so 

 definite a conclusion. 



Schleiermacher's results are obtained by a special and 

 apparently very appropriate method, namely that of measuring 

 the heat conducted through a gaseous envelope from a wire 

 heated by an electric current, and yet his values of k 100 /k Q 

 range from 1*256 to 1*318 for air, from 1*200 to 1*315 for 

 hydrogen, and from 1*485 to 1*584 for C0 2 , and the separate 

 measurements of other experimenters vary in the same 

 manner. Under the circumstances of the case all that we can 

 say is that when molecular force is taken account of in the 

 theory of the conductivity of gases, the theoretical variation of 

 conductivity with temperature is brought within the range of 

 present experimental determinations in a manner which is not 

 possible when molecular force is ignored. The direction in 

 which further experimental work is desirable is that of testing 

 for other compound gases where c is largely variable with 

 temperature, whether k oc cT*/(l + C/T). 



The whole theory of the conduction of heat in gases awaits 

 development ; it has been touched on here in only one aspect, 

 namely that of its dependence on molecular force. 



The last property of a gas which we shall take as being 

 affected by molecular force in a manner hitherto ignored is 

 its characteristic equation. I have shown (Phil. Mag., March 

 1893) that the equation of Van der Waals applies only to 

 the element gases and methane and not to compounds; but if 

 it only applied to a single substance it would still be of great 

 interest in relation to the kinetic theory. In the theoretical 

 deduction which Yan der Waals gave of his characteristic 

 equation (p + a/v*) (v—b) = RT the number of encounters of a 

 molecule was shown to have the important effect of introducing 

 the constant b into the equation, b being (when molecular 

 force is ignored as affecting the number of collisions) equal to 

 four times the volumes of all the sphere-molecules in volume 

 v ; but when the influence of molecular force on the number 

 of collisions is allowed for, then b can no longer be regarded 

 as a constant equal to four times the volumes of the molecules, 

 and the theoretical form of the characteric equation of gases 

 is rather profoundly affected. 



In tracing this effect of molecular force it will be most 

 convenient to reproduce the essentials of Tait's method of 

 presenting the establishment of the characteristic equation 

 for forceless molecules (Trans. Roy. Soc. Edinb. xxxiii. and 



