106 Dr. R. D. Kleeman on the Kinetic 



by p and P„ respectively, we have 



| A =/> + P», 



or wl-27xlO- 20 v / T^ = p+P« (1) 



This is the equation of state of a substance involving the 

 quantities P„, p, '\\ and n. 



If we suppose that the motion of the molecules takes place 

 parallel to three straight lines at right angles to one another, 

 and an equal number move parallel to each line, the number 

 that cross in one direction a plane 1 cm. 2 in area parallel 

 to one of the directions is given by 



nA = F n +p (2) 



It will be of interest to obtain numerical values of n lor a 

 liquid at low temperatures, the intrinsic pressure being then 

 large in comparison with the external pressure. Approximate 

 values for the intrinsic pressure can be obtained from the 



equation p , 



i^ 



where Lj denotes the energy necessary to separate the 

 molecules of a gram of substance by an infinite distance 

 from one another ; when the density of the vapour is -mail 

 in comparison with that of the liquid, Lj is the internal heat 

 of evaporation. This equation is deduced on the supposition 

 that matter is evenly distributed in space, i. e., does not 

 consist of molecules. The values of P« can also be obtained 

 by means of the equation 



P _ Ta 



where a denotes the coefficient of expansion with ris 

 temperature, and /3 the coefficient of compression :;t constant 

 temperature. In deducing this equation, the chang in 

 interna] energy of a molecule with a change in the density 

 of the substance was assumed to be /.ere. Both equations 

 give approximately the same result*. Using the latter 

 equation, the values of // at different temperatures given by 

 equation (2) have been calculated (or four liquid-, and are 

 given in Table I., which also contains the values o£ /> 

 calculated en the supposition that each substance behaves 

 perfect gas, The values of n in the latter case are 



