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Proceedings of Royal Society of Edinburgh. [sess. 
A Direct Electrical Method of Determining Latent Heat 
of Evaporation at the Boiling Point. By A. Cameron 
Smith, M.A., B.Sc., Lecturer in Physics, Heriot Watt Col- 
lege, Edinburgh. Communicated by Professor F. G. Baily. 
(MS. received June 25, 1903. Read July 6, 1903.) 
In this method the liquid of which the latent heat is to be 
determined is contained in a glass vessel which is suspended by a 
fine wire from the hook of a balance, and the amount of electrical 
energy is measured which is required for the evaporation of an 
■observed mass. The electrical energy is supplied by a large 
current passing through a small resistance in the liquid itself. In 
order to reduce heat losses as much as possible, the vessel carrying 
the liquid and the resistance is completely surrounded by a double- 
walled shield, which is filled by the saturated vapour of the liquid 
itself. The estimation of the electrical energy supplied necessitates 
trie measurement of the current, C (amperes), the resistance R 
(ohms), of the heating coil, and the time in seconds, T, during which 
the current flows. It is expressed by C 2 RT joules, and if M be the 
mass evaporated in gms., the latent heat expressed in joules per 
gm. becomes C 2 RT/M. 
The specific heat of the liquid does not enter into the calcu- 
lation. 
Arrangement of the Apparatus. 
Various arrangements have been tried during the last two 
summers, but as these were found to be troublesome to 
manipulate and unsatisfactory in other ways, it will be 
sufficient to describe the form now adopted. This is shown in 
the diagrams : ( 1 ) an elevation with the front half of the shield 
removed and the balance in the position for weighing ; and (2) a 
plan of the same, closed for the process of evaporation. The plan 
