156 Journal of the Mitchell Society. [Dec. 



vapor pressure and of the densities of liquid and vapor, would 

 enable the constant for that substance to be calculated. The 

 equation has, therefore, no connection with the thermody- 

 namical equation 3, but rests independently, partly upon the 

 same and partly upon additional assumptions. 



The results obtained for the twenty-one substances are 

 given later, Tables 1 to 21, in the columns marked Mills. 



In comparing the latent heats so calculated with those 

 obtained from the thermodynamical equation we find that if 

 the five associated subtances be omitted, there are only four 

 determinations in which the results as calculated by the two 

 equations differ by so much as two calories, viz. : di-isobutyl 

 at 0°, normal octane at 0°, chlor-benzene at 270°, and brom- 

 benzene at 30° — in every instance the divergences being at 

 the end point of the Biot curve and thus making it probable 

 that all of these divergences are due to the thermodynamical 

 equation. Excepting stannic chloride, there are only twelve 

 other substances in which the divergence is greater than one 

 calorie. All of these divergences are, marked with an asterisk 

 in the tables. This comparison emphasizes more clearly than 

 is possible in any other way the correctness of the law of 

 molecular attraction we have assumed. 



Accurate calorimetric measurements are exceedingly difficult 

 even under ordinary pressures and it is not too much to say 

 that where equation 4 is applicable, latent heats calculated 

 with its aid will be more accurate than direct measurements 

 of»that quantity, unless very great care is taken in the meas- 

 urements. 



Equations 3 and 4 give with the associated substances and 

 with stannic chloride an agreement which is partial but not 

 complete, divergences being shown at the higher tempera- 

 tures. One or more of the modifying causes mentioned on 

 page 155 may here be operative. 



Equation 5 was deduced by Mr. H. Crompton. Mr. Cromp- 

 ton considers the change in density as if it were due to pres- 

 sure alone, then in order to keep the substance at that density 



