HEAT OF EVAPORATION OF WATER. 321 
I see nothing impossible in this supposition. As stated in Section IT. there is 
sufficient evidence that at low temperatures the capacity for heat decreases with rise 
of temperature. RowLaNp found a minimum indicated near 34°. If therefore the 
capacity for heat increases gradually above some such temperature, but more rapidly 
near 100°, it is quite conceivable that the “‘mean thermal unit” should closely 
approximate to the “thermal unit at 15°C.” Our only experimental evidence to the 
contrary is that given by RecNautr in his paper, “ De la Chaleur Spécifique.” We 
know that his conclusions at low temperatures are incorrect, and [ do not see that 
those at higher temperatures have greater value, for his methods of observation and 
experiment were, in this case, unaltered. 
The matter, of course, can only be cleared up by a direct determination of the 
capacity for heat of water over the range 0° to 100°. 
In the meantime I contend that the evidence in favour of the formula “ total 
heat = 596°73 + °39900” is stronger than that upon which either REGNAULT’s or 
WINKELMANN’S formule are based. 
At temperatures above 100° the values of “the total heat” deduced by formula 
(G3) would be higher than REGNAULT's experimental numbers. The capacity for heat 
of water at higher temperatures is so uncertain, and it has so great an influence on 
the values of L at high temperatures when deduced from the “ total heat” formule, 
that I do not feel that a discussion on these results would at present be of any value. 
The results of the experimental work described in preceding sections, and of the 
evidence brought forward in this section, may be summarised as follows :— 
THE VALUES OBTAINED BY DIETERICI AT 0°, BY REGNAULT AT TEMPERATURES 63° 
To 100° C., aND By GRIFFITHS AT INTERMEDIATE TEMPERATURES ARE (ASSUMING THE 
APPROXIMATE EQUALITY OF THE “‘ MEAN THERMAL UNIT” AND THE THERMAL UNIT AT 
15° C.) CLOSELY REPRESENTED BY THE FORMULA 
L = 596'73 — 0°60100. 
[Note, May 7, 1895.—The suggestion that the “mean thermal unit” does not 
exceed the “thermal unit at 15° C.” has been eriticized as over bold. It is, therefore, 
with peculiar pleasure that I include in this paper a communication which I have 
to-day received from Dr. Joty, to whom I return my sincere thanks for his permission 
to publish it. 
Dr. Joty informs me that he regards his experiments as preliminary; their 
importance, however, is undoubtedly great. They are (I believe) the first experi- 
ments since those of RecNavutt which throw any light on the relative values of the 
two units. It will be seen that (assuming Barront and Srraccratr’s conclusions as 
to the changes in the capacity for heat of water from 0° to 15° C.) the results of 
Dr. JoLy’s experiments indicate that the ratio 
ice in terms of the same unit, by means of the ordinarily accepted comparison of the values of their 
respective thermal units, their difference becomes excessive. 
MDCCCXCV.— A, 2 
