Properties of a Molecule in a Substance. Ill 



certain value. It is very probable that when l x ceases to 

 vary inversely as the density of a substance, P rt ceases to 

 be negligible in comparison with p, since the deviation in 

 each case would be caused by the influence of molecular 

 attraction. 



The particular density of a gas at which l } ceases to vary 

 inversely as the density is therefore of interest. It occurs, 

 according to the equation rj = ^vpl, when the viscosity ceases 

 to be independent of the density. The variation of the 

 viscosity of COo with the density is shown in Table IV., 



Table IV. 



p (mm. Hg.). 



p. 



1 



p (atmos.). 



P- 



* 



•6 



Propor- 



•0001433 1 



43-1 



o-ioo 



•000177 



2-4 



tional to 



1498 | 



607 



•170 



189 



380 



pressure. 



1511 



79-3 



•590 



429 



750 





1524 



111-5 



•800 



683 



which is taken from Weinstein *. It will be seen that the 

 deviations of the viscosity from constancy as the density is 

 increased do not occur abruptly, showing that there is no 

 abrupt change in the field of force surrounding a molecule, 

 as we pass from its centre outwards. 



A General Formula for the Coefficient of Conduction of Heat. 



A general formula for the coefficient of conduction of heat 

 can be obtained along the same lines as that for the viscosity. 

 The amount of heat that crosses 1 cm. 2 per second in a 

 substance, when the heat gradient is 1 degree per cm., is 

 usually defined as the conductivity or the coefficient of con- 

 duction. Consider a molecule moving in the direction of 

 the flow of heat at the standard temperature gradient. Let 

 its temperature be that of the surrounding molecules at the 

 beginning of the motion, and let l 2 denote the distance in the 

 direction of flow of heat it traverses before its temperature is 

 reduced to that of the surrounding molecules. Let t L and t 2 

 denote the temperatures of the molecule at the beginning and 

 end of its journey. The temperature in each case is that 

 corresponding to the minimum velocity of the molecule. 



* Thennodynamik unci KinetiJc der Korper, i. pp. 325-326. 



