[ 38 ] 



II. A Method of Comparing directly the Heats of Evaporation 

 of different Liquids at their Boiling-points. By Miss 

 Doeothy Marshall, B.Sc, and Prof. W . Ramsay, Ph.D., 

 F.R.S.i University College, London *. 



1. ri^HE heat of evaporation of a liquid is usually deter- 

 JL mined by measuring with the aid of a calorimeter 

 the amount of heat liberated during the condensation of a 

 known mass of the vapour ; or conversely, by measuring the 

 heat absorbed during the vaporization of a known mass of the 

 liquid. The former method is the one more frequently 

 adopted, e. g. by Berthelot and by Schiff. 



Apart from the direct experimental difficulties of the method 

 (and these are by no means inconsiderable) , it is open to the 

 objection that it measures, not simply the heat set free during 

 the passage of a given mass of the substance from the gaseous 

 to the liquid condition, but the sum of this quantity of heat 

 with that liberated during the cooling of the liquid from the 

 temperature of its boiling-point to the final temperature of 

 the calorimeter. It is therefore necessary to make a separate 

 determination of the thermal capacity of the liquid between 

 these temperatures. 



Whatever the cause may be, the fact remains that the data 

 regarding heats of vaporization are very scanty. The subject 

 has been worked at by few observers, and the results obtained 

 show very considerable discrepancies. Taking, for instance, 

 the case of ethyl alcohol, the heat of vaporization of a gramme 

 is given as 208 calories by Despretz, 227 by Brix, 202*4 by 

 Andrews, and 208*9 by Favre and Silbermann. It is clear 

 that no method can be really satisfactory that leads to such 

 discordant results. 



2. Professor Ramsay suggested some time ago that it 

 should be possible to compare directly the heats of evapora- 

 tion of two liquids, by raising the temperature of each to its 

 boiling-point (surrounding each vessel with a jacket of its 

 own vapour), and then determining the loss of weight sus- 

 tained by each vessel when a current of electricity was passed 

 through a carbon filament immersed in the liquid. All other 

 conditions being made the same for the two vessels, the ratio 

 of their losses of weight should give the inverse ratio of the 

 heats of evaporation of the liquids. This method of com- 

 parison we have since been engaged in working out ; and 

 having once overcome the initial difficulties of apparatus &c, 

 we have been able to obtain some very satisfactory results, 



* Communicated by the Physical Society : read November 8, 1895. 



