HEAT OF WATEE, WITH EXPERIMENTS BY A NEW METHOD. 
27 
to uncertainty of tlie water-equivalent of the calorimeter and its variation with 
temperature, have been practically eliminated by keeping all the conditions steady, 
so that the observations could be pushed to the limit of accuracy of tempei'atui'e 
measurement. Errors due to dissolved air and to evaporation, which are quite 
appreciable at 40° C., and begin to be serious at 50° C., with an open calorimeter, 
have been minimised by keeping tlie water free from air up to the moment of its 
passage into the calorimeter, where it is completely enclosed and protected from 
evaporation. Without presuming to criticise in detail tlie work of Messrs. Bousfield, 
I maintain that tliese difficulties have not been adequately met in tlieir investigations, 
and would inevitably give rise to constant errors, which could not be detected without 
fundamental variations in the conditions of experiment. While their method may 
undoubtedly be suitable for the purpose for which it was originally devised, namely, 
that of comparing the specific heats of similar solutions over the same range of 
temperature, I cannot admit that it afibrds any promise of exceptional accuracy in 
the determination of the variation of the specific heat O'ver different ranges of 
temperature, which is a much more difficult problem. According to my own work 
and that of Dr. Barxes, the whole variation of the specific heat of water between 
10° C. and 80° C. is less than one-half of 1 per cent., and demands the most accurate 
metliods of investigation. The occurrence of so high a maximum as Ludin’s 
experiments show in tlie neighbourhood of 80° C. is theoretically inadmissible, and 
cannot be reconciled with the work of Beynolds and Moorby, or with the corrected 
results of B;EGNAULT, which are satisfactorily represented by my formula. 
Variation of Specific Heat with Pressure. 
The continuous-electric and continuous-mixture methods both compare the varia¬ 
tions of the total heat of the fluid, li — E-|-pc, under the condition of practically 
constant pressure, or give ratios of the values of the mean specific heat at constant 
pressure over dlfterent ranges of temperature. Below 100° C. the pressure is always 
atmospheric, and the variation of the specific heat with pressure does not exceed 1 in 
10,000 per atmosphere. At higher pressures and temperatures it would be necessary 
to take account of the variation of the specific heat with pressure in comparing 
observations taken at different pressures, since the pressure in the apparatus must 
always exceed the saturation pressure at the highest temperature observed. This 
reduction can be eftected with sufficient approximation by means of tlie thermo¬ 
dynamical relation, 
{dsldp)e = —9{dihi/dd“)p. 
The variation amounts to nearly 2 in 10,000 per atmosphere at 200° C. The quantity 
of heat supplied to water in a boiler, maintained at constant pressure and tempe¬ 
rature, per unit mass of water pumped into it at the same pressure but at a lower 
E 2 
