320 On Molecular Diameters. 



the loss of temperature of the ground by radiation is very 

 small in comparison to the loss by convection, in other words 

 that we gain very little from the circumstance that the 

 radiation is trapped. 



Is it therefore necessary to pay much attention to trapped 

 radiation in deducing the temperature of a planet as affected 

 by its atmosphere ? The solar rays penetrate the atmosphere, 

 warm the ground which in turn warms the atmosphere by 

 contact and by convection currents. The heat received is 

 thus stored up in the atmosphere, remaining there on account 

 of the very low radiating power of a gas. It seems to me 

 very doubtful if the atmosphere is warmed to any great 

 extent by absorbing the radiation from the ground, even 

 under the most favourable conditions. 



I do not pretend to have gone very deeply into the matter, 

 and publish this note merely to draw attention to the fact that 

 trapped radiation appears to play but a very small part in 

 the actual cases with which we are familiar. 



XXV. Molecular Diameters. By William Sutherland *. 



GREATER absolute precision and better mutual con- 

 sistency were introduced into the measurement of 

 molecular diameters by the kinetic theory of gases, when 

 Jeans applied electrical data to give the number of molecules 

 of a gas in a cm. 3 under normal conditions (Phil. Mag. [6] 

 viii. 1904, p. 692). But in his calculations he took no 

 account of the effect of cohesional forces in the viscosities, 

 conductivities, diffusivities, and collisional virials of gases 

 which he used in the calculation of molecular diameters. 

 Now in " The Viscosity of Gases and Molecular Force " 

 (Phil. Mag. J5] xxxvi. 1893, p. 507) it was shown that if 

 2a is the diameter of a molecule, and C a parameter pro- 

 portional to the mutual potential energy of two molecules in 

 contact, T denoting absolute temperature, the molecules 

 behave as if devoid of attractive force, but enlarged so that 

 (2a)-' is replaced by (2a 2 ) 3 (l + C/T) . Thus the temperature 

 law of the viscosity of a natural gas is 77 <x T*/(l -f C/T), instead 

 of the law rj cc T* which holds for the ideal perfect gas. On 

 this account the quantities given by Jeans as the diameters 

 of molecules 2a, are in reality 2a(l-f C/T)*. By means of 

 the values of C and with T = 273 it is easy to obtain the true 

 values of 2a. In the Landolt-Bornstein-Meyerhoffer Tabellen 

 values of C for many of the gases in the list of Jeans are 



* Communicated bv the Author. 



