COXTRIBUTIONS FROM THE CHEMICAL LABORATORY OF 

 HARVARD COLLEGE. 



A METHOD FOR DETERMINING HEAT OF EVAPORATION 

 AS APPLIED TO WATER 



By Theodore W. Richards and J. Howard Mathews. 



Presented by Theodore W. Richards. Received December 24, 1910. 



Among usual calorimetric measurements none has been in the past 

 less satisfactory than the measurement of heat of vaporization. The 

 methods employed have been almost as untrustworthy as they have 

 been numerous. The results for water are indeed not very divergent, 

 but in most other cases there is little or no concordance. In the case 

 of ethyl formate, for example, the variation in the values given by dif- 

 ferent experimenters is over ten per cent ; in the case of ethyl acetate, 

 the extreme difference is over thirteen per cent. These cases have been 

 selected because they concern substances carefully investigated by 

 many observers ; less carefully studied cases might have been selected 

 which show even greater discrepancies. Evidently most of the results 

 are in error ; but which are the correct ones ? The newest values are 

 by no means necessarily the best, for wide discrepancies have appeared 

 in the most recent work. Part of the errors were undoubtedly due 

 to impurity in the materials, and some to faults in the methods. 



Desiring really to know the heats of evaporation of a few liquids, we 

 felt hopeless concerning our ability to select among these discordant 

 figures. New research was evidently needed, involving great care in 

 purification of material, and careful choice of the best details of experi- 

 mentation. The present paper contains an account of the evolution 

 of a satisfactory method. 



A brief account of previous methods may well precede the descrip- 

 tion of our own experimental work, as this was based partly on the 

 successes and partly on the failures of others. 



The methods used for measurements of heats of evaporation are of 

 two classes. In one the energy used in the process of vaporization is 

 measured ; in the other the energy given up by the condensation of 

 vapor is evaluated; each procedure should of course yield the same 



