HEAT OF EVAPORATION OF WATER. 269 
knowing the nature of the glass, he simply reduced them to the air scale by the table 
above referred to. 
Throughout the low temperature determinations the temperature of the calorimeter 
fell during an experiment through about 5°°6 (in the greatest case 5°°761), and thus 
an error of 0°01 in his thermometry would cause a difference of more than 1 in 600 
in the results. At higher temperatures, however, the average rise in the temperature 
of the calorimeter exceeded 12° and thus the effect of any such thermometric error 
would be considerably reduced. 
Again, the observations of the change in temperature of the calorimeter at low 
temperatures were always taken on a falling thermometer. I have in a previous 
paper (J., p. 442) expressed my disbelief in the value of any observations of mercury 
thermometers when their temperature is falling. I am not alone in this opinion,* 
and it has been confirmed by subsequent experience. I am sure that inaccuracies of 
a much larger order than 0°01 would have presented themselves from this cause 
alone, and the great divergences observable amongst REGNAULT’S individual experi- 
ments at low temperatures is I have no doubt partly attributable to this cause.t In 
all his experiments at higher temperatures, however, his thermometers were rising 
and the discrepancies between individual observations were much less marked. 
_ It will be noticed that the various sources of error which have been enumerated 
either disappear or are much diminished at the higher temperatures. 
Again, the ever-recurring difficulty with regard to the specific heat of water 
presents itself. The correction is not so simple as has been assumed by those who 
have merely applied ReGNAULT’s own formula for the specific heat of water to the 
expression for the total heat of steam, for the correction would have to be applied 
during the reduction of each separate experiment, as the quantity of heat absorbed 
by the calorimeter and contents when warming or cooling through a degree of 
temperature would vary according to the mean temperature of the range and the 
resulting correction would, I believe, be greater than is usually supposed. 
There appears to be but little doubt that ReG@NAuLt’s expression for the changes 
in the specific heat of water is inaccurate. As far as I know, RowLanp (1877), 
Bartout and StRAccrati (1889), and myself (1892) are the only observers who have 
seriously attacked this difficulty since the time of ReGNAULT, and all agree in one 
conclusion, viz., that the specific heat of water diminishes as the temperature rises to 
20°, and the methods of experiment employed by these observers were so entirely 
different that their agreement in this matter carries great weight. We cannot, 
therefore, accept without question REGNAULT’s conclusions as to the changes at 
higher temperatures. The magnitude of the correction involved may be illustrated, 
as pointed out by DirrTericr himself, by the following example. If we assume 
* Scuusrer and Gannon, Communication to the Royal Society, Nov. 22, 1894. 
+ The comparatively slow rate of stirring would also tend to make the temperature measurements 
uucertain. 
