148 HEAT. 



In the opposite direction let mass ^ pass upwards at the temperature 



dO 

 of the layer CD a distance L below EF, that is, at temperature 6 - L . 



The excess of energy carried through EF from above over that re- 



0V 

 turned from below is therefore the same as if mass -5- were cooled 



d.9 



through 2L ' and if K V is the specific heat of constant volume, this will 

 dx 



be equal to 



pv de_ KvP vi. do 



"6 clx~~3~ dx 



If K is the conductivity for heat of the gas, the quantity of heat 



dO oVL pVL 



passing down per second is K -,-, whence ~K. = zK V) or putting TT~ =">} 



the coefficient of viscosity, JL = ^K V) which is independent of the density 



FIG. 78. 



of the gas, a result predicted from the kinetic theory by Maxwell and 

 subsequently verified by experiment. 



The Diameter of the Molecules and the Number of Molecules 



per Cubic Centimetre. The value of the molecular diameter, or rather 

 the radius of the sphere of action, was first calculated by Loschmidt in 

 1865. 



Taking s as the radius of the sphere of action as defined on p. 143, 



- may be taken as the radius of each molecular system when two 

 2 



molecules are at their nearest approach with their centres s apart. Then 

 the volume of a singular molecular system is of the order 



The total volume of the molecules in 1 c.c. is therefore . . 



If we could suddenly destroy the translatory motion of the molecules 

 in 1 c.c., still keeping the molecular systems unchanged in themselves, 

 they would simply fall to the bottom of the space and occupy a volume 



N7T.S 3 



of the order ^-. If we imagined them all exact spheres, piled up like 



