HEAT OF EVAPORATION OF WATER. 271 
and if these points are borne in mind they may serve to explain some of the con- 
trivances which might otherwise appear uncalled for. 
If the vessel in which the evaporation is taking place is kept at a constant tem- 
perature, we are independent of the capacity for heat of it and its contents; we also 
dispense with the measurements of changes of temperature. Thus, if matters be so 
arranged that the loss and gain of heat throughout an experiment are balanced, many 
frnitful causes of error are avoided. Of course, the actual temperature of the calori- 
meter during evaporation must be determined, but a small error here is of little con- 
sequence. The change in the value of L (when L is the latent heat of evaporation of 
water) is small as compared with the changes in #. In fact, an accuracy of an order 
of ;'5 of a degree would be sufficient when determining the actual elevation. 
The heat was supplied to the calorimeter by means of a wire whose ends were kept 
at a constant potential difference. The thermal balance could be maintained in one 
of two ways, 
(1) If the heat supply was too great, the electric current could be temporarily 
stopped : or, the rate of evaporation of the water increased. (The latter was 
the method that I generally adopted.) 
(2) If the cooling was too rapid, the only mode of maintaining the balance was (in 
the apparatus about to be described) to reduce the rate of evaporation. 
The water to be evaporated was placed in a small silver flask, connected with which 
was a spiral coil of silver tubing 18 feet in length. Both flask and spiral were within 
the calorimeter, and the water-vapour, after passing through the spiral, emerged from 
the apparatus at the temperature of the calorimeter. Surrounding the flask, and 
between it and the spiral, a coil of platinum silver wire was arranged, and flask, 
spiral and coil were entirely immersed, in aniline during my preliminary experiments, 
subsequently in a certain oil of which an account will be given later. 
The calorimeter (which was filled to the roof with the aniline or oil, and the equality 
of temperature maintained by rapid stirring) was suspended by glass tubes within a 
steel chamber, whose walls were maintained at a constant temperature. So long, 
therefore, as the calorimeter and the surrounding walls were at equal temperatures, 
there was no loss or gain by radiation, &c. If during an experiment the temperature 
of the surrounding walls changed, the method of experiment involved a corresponding 
change in the temperature of the calorimeter, and, therefore, some loss or gain of heat 
would be experienced. The apparatus was so designed that any such change in 
temperature was extremely small (in no case amounting to 7p), yet, in order to 
estimate the loss or gain, it was necessary to know approximately the capacity for 
heat of the calorimeter and contents. 
Small differences between the temperature of the calorimeter and the surrounding 
walls would, during an experiment, be of no consequence, provided that the oscillations 
were of such a nature that the mean temperature of the calorimeter was that of the 
