270 MR. E. H. GRIFFITHS ON THE LATENT 
Row.anv’s value for the specific heat of water at 15° C., the difference between the 
thermal unit at 15° C. and “the mean thermal unit” over the range 100° to 0°, 
amounts (if we accept Drererict’s interpretation of REGNAULT’s values) to nearly 
13 per cent.,* and would reduce the value of L at 0°, as given by Ruanautt’s formula 
(viz., 606°5), below the value found by Drererictr (598°8).+ 
The tacit assumption amongst physicists that the discrepancies arising from doubts 
as to the value of the thermal unit are so trivial that they may be disregarded is, as 
shown by the above example, much to be regretted, and the many efforts to deduce 
specific volumes, etc., from the equation J = L/T (s’ — s) dp/dT, shows how frequently 
this difficulty is ignored. It is strange that, although so much attention has been 
devoted, during recent years, to the exact determination of various physical units, so 
little has been done with regard to this extremely important fundamental constant. 
The experimental difficulties are not so great as to prevent all progress, and 
I venture to appeal to the Royal Society to consider this matter ; indeed, I would 
go so far as to express my personal belief that the method of measuring small 
differences of temperature indicated in a paper read before the Physical Society 
last October removes many of the difficulties which have hitherto barred the way. 
It is, I think, evident that we are not justified in concluding that our knowledge 
of the value of the latent heat of evaporation of water at low temperatures is sufficient. 
It has been already shown that the effect of most of the causes of error above 
enumerated diminishes at higher temperatures; and a study of Reenavtt’s Tables I. 
to IIT. will confirm the conclusion that we may regard the results at those tempera- 
tures as of greater accuracy. Even at higher temperatures, however, the difficulties 
with regard to the measurement of differences of temperature and of the capacity for 
heat of water present themselves. 
I can find no record of experiments by any observer at temperatures between 16° 
and 65° C. 
The above considerations are, I think, sufficient to indicate the necessity of a 
re-determination of the latent heat of evaperation of water, at all events at low 
temperatures. 
Section IIJ].—Derscrierion or THE METHOD. 
I was anxious, if possible, to devise a method of such a nature that my results would 
not be appreciably affected by 
(1) errors in thermometry, 
(2) changes in the specific heat of water, 
(3) the capacity for heat of the calorimeter, 
(4) loss or gain of heat by radiation, &c., 
* At the end of this paper I give figures which lead to the conclusion that the difference between the 
‘* mean thermal unit’? and the “thermal unit at 15°” 
+ Dreterict, ‘ Wied. Ann.,’ vol. 37, 1889, p. 506. 
is less than is usually assumed. 
