RICHARDS AND MATHEWS. — HEAT OF EVAPORATION. 



519 



be described presently, and a striking systematic irregularity was 

 observed in the results. The heat of vaporization of benzene, for 

 example, appeared to be about 81) calories when 25 grams took 14 

 minutes to evaporate, but as much as 93 calories when the time was 

 shortened to 2..') minutes, with corresponding results for intervening 

 rates. The results are plotted in the accompanying diagram. 



Figure 2. The Effect of Speed of Vaporization on the Results. (Benzene 

 in the Vaporizer sho\\Ti in Figure 1.) 



In the direction of ordinatos are plotted the observed values for the heat 

 of vaporization of a gram of benzol in small calories; in the direction of ab- 

 scissae are plotted the times (in fractions of a minute) needed for the vapori- 

 zation of that quantity. 



The ordinates represent heats of vaporization in calories, and the 

 abscissiB, the time in minutes required to vaporize one gram. By a 

 comparatively short linear extrapolation we obtain the value 94.1 cal. 

 as the heat of vaporization of benzene. It will be shown in a subse- 

 quent paper that this value is very near the most probable value ob- 

 tained afterwards with better apparatus. Water was found to exhibit 

 precisely the same phenomenon. Previous investigators have not 

 taken into consideration this variation of result produced by varying 

 the rate of distillation, and the oversight undoubtedly accounts for 

 much of the wide variation in the published results. 



Consideration of the various possible complications which might 

 bring about this time-effect led to the conclusion that it was probably 

 due to premature condensation between the vaporizer and the con- 

 densing coil, and the consequent loss of heat from the vapor thus con- 

 densed. Evidently such a loss must be directly proportional to the 

 time required for the vapor to pass through the zone of premature 

 condensation. Further, in accordance with Newton's Law of Cooling, 

 the loss of heat should be proportional to the difference in tempera- 

 ture between the boiling point of the substance and the environment 

 of this zone, — a conclusion later verified by the facts. 



Attempts were next made to eliminate as far as possible the zone of 

 premature condensation. The vaporizer was set into an asbestos shield 



