HEAT OF EVAPORATION OF WATER. 303 
rate of rise due to the “ stirring supply ’—firstly, when no air was passing ; secondly, 
when the air was passing through the dry flask and tubes and I could detect no 
difference. 
So long as the thermal balance is maintained with the same potential difference, 
the rate of evaporation must be the same, whatever method is adopted. I cannot 
see that the external work to be done is likely to alter because the space above the 
water is de-saturated by passing a gas instead of removing the saturated vapour by 
an exhaust, and this view is borne out by the close agreement between the means of 
those groups and my later experiments. 
There is one curious fact which may possibly throw some light on the matter. I 
performed two experiments (August 22 and September 13), passing nitrogen instead 
of air. Of course I had at that time no means of judging of the comparative value of 
the different experiments, and therefore was ignorant of the close approximation of 
the nitrogen values and my final ones. Had I then been aware of the coincidence, I 
should have continued the nitrogen experiments in order to ascertain if the agreement 
was fortuitous. 
The close agreement is shown in the following table :— 



TABLE X. 
Temperature. | Results of nitrogen experiments. | Final values. 
0 | 
24-96 581°68 (1 cell) | 581-73 
39-98 572°72 (2 cells) | 572°69 

When time allows, I propose to repeat these nitrogen experiments, in the hope 
that some light may thereby be thrown on the matter; in the meantime, I am 
unable to suggest any sufficient explanation of this phenomenon. 
Section X.—THE MertHop FINALLY ADOPTED. “ EXHAUST” EXPERIMENTS. 
A study of the results obtained from the experiments described in the last section 
led me to the conclusion that it was necessary to seek some other method of 
attacking the problem. I have not described many of the precautions I gradually 
adopted during those experiments, but since none of them had produced any appre- 
ciable improvement it was evident that the real cause of the irregularities had not 
been ascertained. Even the entire change involved by the replacement of aniline by 
oil, the removal and refitting of the calorimeter, &c., appeared to have had no effect 
of any kind. 
__ I determined, therefore, to adopt an alternative method of producing evaporation, 
viz., by reducing the total pressure below the pressure of the saturated vapour. It 
