84 Mr. W. Sutherland on the Law of Attraction 



Thomson and Joule's paper in the Phil. Trans. 1862) ; in the 

 third the calculated cooling effect - — J- — ; in the fourth the 



value of 6 or actual cooling effect minus z — j ; and in the 



fifth the products T0. 



Air. 



Absolute 



temperature, 



T. 



Total actual 

 cooling effect. 



Cooling effect, 



pv—p'v' 



Js 



Difference of 



the two cooling 



effects, 0. 



Product, 

 T0. 



273-0 

 2801 

 3125 

 365-8 



o 



•92 

 •88 

 •75 

 •51 



o 



•320 

 •294 

 •183 

 •046 



•600 



•586 

 •567 

 •464 



164 

 164 

 177 

 166 



Thus within a range of nearly 93° the product TO is 

 practically constant, showing that the cooling effect due to 

 increase of molecular potential energy is inversely proportional 

 to the absolute temperature of the gas. The erratic number 

 177 is eliminated from the following table, in which, instead 

 of the actual, experimental total cooling effects are used, those 

 which Thomson and Joule calculated in the light of the fact 

 that variation of the total cooling as the inverse square of the 

 absolute temperature was the clear meaning of their experi- 

 ments as a whole : — 



Absolute 



temperature, 



T. 



Calculated 



actual 



cooling effect. 



Cooling effect, 

 pv—p'v' 



Difference of 



the two cooling 



effects, 9. 



Product, 

 T0. 



273-0 



o 



•92 



•320 



•600 



164 



280-1 



•87 



•294 



•576 



161 



312-5 



•70 



•183 



•517 



162 



365-8 



•51 



•046 



•464 



166 



With the relation 



e^ 



thus established, it is not 



difficult to see that it means that the attraction between any 

 two molecules of the gas is proportional to the product of their 

 masses, and inversely proportional to the fourth power of the 

 distance between them. 



For the mutual potential energy of the two molecules is 

 inversely proportional to the cube of the distance between 



