HEAT OF EVAPORATION OF WATER. 265 
depend entirely on the constant which gives the relation between the quantity of 
heat and the mass of expelled mercury. DieETERIct in this matter shifts the respon- 
sibility on to other shoulders. His only reference to the subject is as follows :— 
“Diese sind gemessen in mittleren Calorien, also in dem hundertsten Theile 
derjenigen Wiirmemenge, welche ein Gramm Wasser von 0° auf 100° erwiirmt, und 
zwar liegt der Berechnung der Mittelwerth der Beobachtungen von BuNSsEN, 
ScHULLER und WartTHA und VELTEN,* zu Grunde dass einer mittleren Calorte 
15°44 mg. Hg. entsprechen.”+ 
Now, Bunsen by assuming his own value of this constant obtained 80°025 as the 
latent heat of fusion of ice, and the marked difference between this number and that 
obtained by REGNAULT (79°24), requires explanation. This discrepancy is greater 
than appears at first sight, for the ‘‘mean thermal unit” (over the range 100° to 0°) 
adopted by BuNSEN is supposed to be greater than the “thermal unit at 15°” 
adopted by REGNAULT during his researches into Latent Heat, and thus the divergence 
would be increased if both were expressed in terms of the same thermal unit. 
The doubt introduced by the above considerations is due to uncertainty regarding 
the comparative magnitude of the different thermal units and does not affect the 
value of Dirrericr’s experiments, although it renders his conclusions somewhat 
uncertain. 
There can be but little doubt that the mass of mercury expelled from a Bunsen 
calorimeter by the subtraction of a definite thermal unit is a quantity that can be, and 
doubtless will be, determined with accuracy, and if any correction on the conclusions 
arrived at by BuNSEN, SCHULLER and WaRrHA, and VELTEN is found to be necessary, 
it can be applied to the values of the latent heat of evaporation at 0° obtained by 
DIeTERICI. 
This case well illustrates a matter to which I have before endeavoured to call 
attention,{ viz., that a mistake in thermometry is a fatal error in experimental 
work. It is impossible to correct the conclusions arrived at by the investigator, 
however our knowledge of thermometry may increase, since we should require to have 
in our possession the actual thermometer used by the observer, together with a full 
knowledge of the circumstances under which it was observed. As before remarked, 
Drerericr’s results were independent of thermometry, hence their peculiar value. 
ReeNav.t’s formula for the “ total heat of steam” has been so generally accepted, 
and the experiments upon which he founded it are so justly considered as examples of 
that singular skill and ability for which all his work is distinguished, that it is with 
diffidence that I venture to offer criticisms on his methods or conclusions. I would 
repeat that Recnautr himself evidently attached less importance to his determina- 
* The actual values obtained by these observers were as follows:—Bunsen, 15°41; Scuutier and 
Warraa, 15-442; Vuuren, 15°47 mg. ‘Wied. Ann.,’ vol. 33, 1888, p. 439. 
+ ‘Wied. Ann.,’ vol. 37, 1889, p. 499. 
t ‘Science Progress,’ April, 1894. 
MDCCCXCV,.—A. 2M 
