HEAT OF EVAPORATION OF WATER. 267 
When I give the results of my experiments, I think that I shall be able to show 
that WINKELMANN in his desire to lower the value of L at low temperatures has 
considerably overshot the mark, and that in deducing the values at, or near, 0° from 
experiments above 65° he has carried the method of extrapolation beyond due 
bounds. At present, however, J will only consider his reasons, with which I agree in 
the main, for rejecting REGNAULT’s determinations at lower temperatures. 
(1) There is no doubt that Reanavtt’s formula does not give the mean result of 
his experiments at low temperatures. In all, he performed twenty-two experiments 
at temperatures below 65° (Table IV., ‘Mémoir. de l’Acad.,’ xxi., 1847), and the 
mean value given by these twenty-two experiments differs from that given by his 
formula by_1°8. 
(2) The experiments above referred to were performed in a different manner from 
those at the higher temperatures. The water to be evaporated was placed in a 
spiral within the calorimeter, and the pressure reduced until the water boiled, the 
vapour being condensed in a vessel surrounded by ice. Recnavir deduced the 
temperature of the water when evaporating by observing the pressure of the vapour 
in the condenser—hence, as REGNAULT himself says, ‘‘ It is probable that the elastic 
force observed on the barometric manometer is decidedly less than the mean pressure 
at which the vapour is distilled,” and thus the evaporation is taking place under a 
greater pressure and at a higher temperature than that given by his Table IV. 
Again, the temperature of the saturated vapour is sensibly beneath the tempera- 
ture of the calorimeter, and so lowers the temperature of the calorimeter more than 
would be done by the evaporation alone. 
In the case of other liquids, REGNAULT made a correction for the heat abstracted 
by the vapour while passing out of the calorimeter, but he did not apply this correc- 
tion in the case of water. It is true that no special arrangements were made to 
warm the vapour to the calorimetric temperature (as was done by coils with the 
vapours of other liquids), but there can be no doubt that the vapour must have 
abstracted heat from the walls of the calorimeter. 
Let & be the specific heat of the vapour, ¢ the temperature at which evaporation 
takes place, t, and ¢, the initial and final temperatures of the calorimeter, then the 
heat per unit mass absorbed should be £ {3(¢,-+¢,) —t}; at the same time this is 
not a correction that can be applied with certainty. 
(3) At these low temperatures a small difference in the pressure of the vapour 
corresponds to a considerable ditference in temperature; thus, if the temperature of 
the water is deduced from the pressure in the receiver, the error may be considerable.* 
The effect of all these errors would be to make the value of L given by REGNAUL1’s 
experiments too great. 
The above criticisms do not apply to the experiments at the higher temperatures. 
* For example, a difference of 0°4 millim. at 4° would correspond to a difference of 1°C, 
2M 2 
