264 MR. E. H. GRIFFITHS ON THE LATENT 
by Reenavtr and Diererici, for none of the other observers appear to me to have 
devoted as much care and attention to the matter. In some cases, ¢.g., ANDREWS 
and BERTHELOT, we have records of only a few observations evidently undertaken not 
so much with the object of obtaining an accurate determination of the latent heat of 
evaporation of water, but rather for purposes of comparison with other liquids. In 
other cases there are not sufficient details as to the thermometry, the unit of heat 
adopted, &c., to render a close criticism of any profit. 
Again, DieTErRIct is the only observer who has made direct experiments at tempera- 
ture 0° ©. REGNAULT’s observations extended from 63° to nearly 200° C., and he also 
performed a number of experiments where the temperature of the vapour was between 
— 2° and + 16°, although this last group must be regarded (and I think was regarded 
by RecGnavtt himself) as of less value than his observations at higher temperatures. 
The method employed by Dirrerici was in principle very similar te that adopted 
by me and described in subsequent pages.* 
The heat required for the evaporation was abstracted from water at 0° and the 
amount of heat deduced from the quantity of ice formed. The advantages of such a 
method (upon which [ shall more fully dwell when I describe my own work) are as 
tollows :— 
(1.) No change of temperature takes place, thus all difficulties connected with the 
capacity for heat of the apparatus and its contents are avoided. 
(2.) The observer is almost entirely independent of thermometry—an advantage 
which, to my mind, it is almost impossible to overestimate. 
DIETERICI’s experimental results, as stated in Table I. (supra), vary from 595°52 to 
598°84, but it should be noticed that both these extremes occur in his Table I. He 
afterwards made what he considered to be improvements in the apparatus, and the 
extreme values of his last 13 experiments (see his Tables II., III., IV.) are 595-74 and 
597°29. His mean result is 596°80, and I would particularly draw attention to his 
last two experiments, where he very greatly increased the rapidity of evaporation by 
suddenly opening the communication between his evaporating vessel and a condenser 
containing sulphuric acid in which the pressure was reduced as far as possible by 
means of a mercury pump. These two experiments give respectively 597°07 and 
596°68, The agreement between the individual experiments throughout the whole 
series leaves little to be desired, and, if it were not for one doubt, I would without 
hesitation accept those results as conclusive; but I am afraid, for the following 
reasons, that our knowledge is not yet sufficiently exact. 
The quantities of heat were deduced by measuring the mass of mercury expelled 
from the BuNSEN calorimeter during the formation of ice, and the results, therefore, 
* My own experiments described in this paper were completed before the work of Dinrerici came to 
my notice, and the close similarity between the general principles adopted by both of us is a matter of 
chance, not of design. Had I previously perused his paper, I should have been saved much time and ° 
many preliminary experiments. 
