1902 - 3 .] Determining Latent Heat of Evaporation. 457 
Evaporation . — At an observed time the current is switched into 
the calorimeter. Observations of the current are now taken every 
minute. This is best done by keeping the galvanometer key 
always closed (if only a small current is flowing through the 
galvanometer). The deflection on the galvanometer scale corre- 
sponding to one division on the potentiometer dial is found once 
for all, and the actual deflection at the minute used as a correction 
on the dial reading. With a current varying uniformly in one 
direction, it is not difficult to anticipate the correct dial position, so 
that the correction may be often zero. No attempt is made to 
keep the current constant by altering the series resistances during 
■an experiment. 
Simultaneously with these readings it is necessary to check the 
balance for the Clark from time to time, say every five minutes. 
When the point of balance for the Clark is not that originally 
arranged (that corresponding to the temperature), no attempt is 
made to restore the original balance, but the point of balance 
merely recorded and allowed for in calculating the current. The 
variation in the reading is never greater than that corresponding 
to ’0003 volt. 
The time of switching off is noted. 
Second Weighing . — The probable weight has been approximately 
arranged, and, (the agate being raised), as soon as evaporation 
has entirely ceased (the balance become steady), the exact weight 
is noted as before. 
Advantages of the Method. 
1. In nearly all methods the total heat is measured and the 
latent heat deduced from an assumption of the mean specific heat. 
In this method the latent heat is determined directly. The same 
applies to the method of Dieterici ( Weid . Ann., 37 (1889), pp. 
494-508), which, however, is only applicable to the temperature 0°, 
and to the method of Griffiths {Phil. Trans., 186 (1895), A., pp. 
261-341), which is only possible for temperatures not very far 
from those of the room. 
2. The latent heat is obtained by an electrical measurement, 
which may be made more accurately than is possible when the 
results depend on the reading of an ordinary mercury thermometer. 
PROC. ROY. SOC. EDIN. — YOL. XXIY. 30 
