igo4\ Mills — Molecular Attraction. 155 



tion of this equation to twenty-one substances, and only for 

 the sake of comparison repeat a portion of the results in the 

 tables which follow. (See Tables 1 to 21, under the heading 

 Ther.) It must be borne in mind that although these results 

 are accurate where the data is correct, yet errors in the meas- 

 urements may, and oftentimes certainly do, (because of the 



8P 

 method necessarily followed for obtaining the -p=-) produce 



far greater proportional errors in the result. 



Equation 4 is derived by a simple transposition from equa- 

 tion 2 of the second paper. The assumptions upon which 

 that equation is founded and evidence bearing- upon the equa- 

 tion, have been discussed in the preceding- paper and here we 

 would only summarize by saying- that this equation rests upon 

 the belief that the total kinetic energy of a molecule of a 

 liquid and of its vapor, at the same temperature, are the same; 

 and upon the further assumption that the entire latent heat 

 of vaporization is expended in overcoming- the external pres- 

 sure and in separating- the molecules ag-ainst the action of an 

 attractive force varying- inversely as the square of the 

 distance apart of the molecules. The equation is not appli- 

 cable {a priori) if (a) the number of molecules change owing 

 to dissociation or decomposition; or if (£) the molecules are 

 not evenly distributed throughout the space occupied by 

 them; or if (c) for any reason the attraction between these 

 molecules varied with the temperature. 



The constant // for the twenty-one substances examined 

 are given in Table 25 of this article under the heading // and 

 while the constants there given were obtained by a compari- 

 son of this equation with the thermodynarnical results 

 obtained by use of equation 3, it is easily seen that such a 

 method of obtaining the constant is, we might say, inciden- 

 tal, and was only adopted for the sake of accuracy and con- 

 venience. One accurate measurement at any temperature of 

 the latent heat of vaporization of any substance to which the 

 equation is applicable, together with a measurement of its 



