82 Journal of the Mitchell Society. [Nov. 



Expressing the above belief in a different form, we may 

 say that the energy necessary to change a liquid into a gas 

 must then be spent solely in overcoming the external pressure 

 and in altering the distance apart of the molecules. (Unless 

 the molecule breaks apart also or nears the point of disrup- 

 tion.) 



Denoting the energy spent in overcoming the external pres- 

 sure by K x , this energy can be calculated from the equation, 



[1] E I = 0.0 4 31833 P(V-z>) cals., 



where the unit calorie is from 15° to 16° C, P is the pressure 

 in millimeters of mercury, V and v are the volumes before 

 and after expansion. To obtain the constant, 0.0 4 31833, we 

 used the values: density of mercury, 13.5956; Rowland's 

 value of the therm, corrected by Day, at 15° to 16° C, 41880000 

 ergs, as unit; and gravity taken as 980.5966. 



Denoting the total latent heat by L, we have L— E x as the 

 energy spent in overcoming the molecular attraction at any 

 particular temperature. 



On the further assumption: 



5. That the molecular attraction varies inversely as the 

 square of the distance apart of the molecules, the equation 7 

 (p. 212) of the original paper was derived, which equation 

 readily takes the more convenient form, 



[2] rir-rg- = constant ' 



for any particular substance, where L — E x is the internal 

 latent heat of vaporization, and d and D are the densities of 

 liquid and vapor at any particular temperature. 



With regard to this equation, we will here say that it was 

 designed to test the assumption advanced in 5. Had it failed 

 to produce a constant or some function of the temperature, 

 the author hoped to substitute 5 by some other distance func- 

 tion of the attraction, obtain the formula similarly, and thus 

 repeat until the correct assumption was made. 



As to the mathematics by which it was derived, the 



