72 THE ROYAL SOCIETY OF CANADA 



Heats of Evaporation. 



Clausius has shown that the heat of vaporisation of a liquid may 



be calculated from the equation, 



I dP W t dP , 



— — = =r=^, where ■— represents the change of vapour pres- 

 rdl Kk dl 



sure with temperature, P the pressure, T the temperature on the 



absolute scale, R the constant of the gas equation, and W the heat 



of vaporisation of one gram molecule of the liquid. 



The value of W at atmospheric pressure calculated from the curves 



(Fig. 3) and the direct determinations of Elliot and Mcintosh are given 



in Table 6. 



Table 6. 



Two points noticed during the progress of this work may be 

 recorded. A paste of carbon dioxide and ether gives a remarkably 

 constant temperature of — 78-2 at atmospheric pressure; but if 

 very cold ether be used the temperature may fall several degrees. 

 Hydrogen iodide distilled in a vacuum and solidified by carbon 

 dioxide is a glass. On warming a few degrees, crystallization begins 

 and spreads through the whole mass. 



We .wish to express our thanks to Dr. F. M. G. Johnson for the 

 use of a sensitive spiral and for many practical hints which enabled 

 us to avoid mistakes in its use. 



Macdonald Chemistry and Mining Building, 

 McGill University, October, 1913. 



