HEAT OF EVAPORATION OF WATER. 325 
calorimeter and obtained a higher value, but it was in an early and very defective 
form of the instrument, and an error of positive sign, as I afterwards found, very 
certainly obtained in those experiments. 
Considering this number in the light of Mr. Grirrirus’ remarks, it certainly 
supports his contention that REGNAULT made an error of excess in his value of the 
mean calorie—0° to 100°. The above number is however, even less than the value 
supposed by Mr. GrirrirHs to be the true number. For, as I understand, 
Mr. Grirrirus’ L is calculated on the calorie at 15° C. as unity. If this is also 
—as Mr. GRIFFITHS suggests as probable—in close agreement with the mean calorie, 
0° to 100°, then the mean specific heat from 12 to 100 should come out only a very 
little less than unity. In fact, by plotting Barroxt and Srracctatt’s observations 
below 15°, we can estimate what the mean specific heat from 12° to 100° ought to be 
if the mean from 0 to 100 is the same as the specific heat at 15° and both equal 
unity. A rough estimate gives this to be 0°9995. 
My value is therefore too low to be in harmony with the supposition that the mean 
calorie and the 15° unit are zdentical. The value of the latent heat of steam is, of 
course, involved, for the steam calorimeter can only give a ratio, and, if the number 
now obtained is correct, it follows that either the latent heat assumed is too low, or 
the specific heat of water is even lower than it is supposed to be, or possibly both are 
somewhat incorrect. 
J. JOLY. 
Physical Laboratory, 
Trinity College, Dublin. | 
Section XII.—Tue Density anp Sesciric VotumME ofr SATURATED WATER- 
VAPOUR DEDUCED BY MEANS OF THE THERMODYNAMIC EQUATION FROM 
THE VALUES oF L GivEN By ForMmULA (G)). 
WINKELMANN (as previously stated) assigns to what he terms the “theoretical 
density ” of water-vapour the value 0°6225 (air = 1). He gives, however, no informa- 
tion regarding the data for this statement. 
The most recent investigation of the comparative volumes in which oxygen and 
hydrogen combine is that by Scorr, whose conclusions are as follows :* “That 
100,000 volumes of oxygen unite with 200,245 ‘volumes of hydrogen to form water. 
Applying this to the density of oxygen found by Lord Rayueien to be 15°882, we 
get for the atomic weight of oxygen 15°862.” 
In close agreement with this conclusion we have—- 
Ditmar . . Pa ee PL SECGG 
Cooke and Ries A) ici bales rabe ig Sacha al pyre A) 
* Phil. Trans.,’ A, 1893, p. 567. Also ‘ Science Progress,’ August, 1894. 
ZT? 
