136 
PROFESSOR HUGH L. CALLENDAR ON 
subtracting the constant quantity '0056 from his values of the specific heat, in 
order to make them agree with the curve of variation deduced from the present 
investigation at a temperature of 60° C. Thus modified, the formula for the specific 
heat s at a temperature t is as follows :— 
From 60° to 200° C. s = '9944 + '00004* + *000,000,9« 2 . . . (l). 
Regnault’s formula for the mean specific heat would require further modification, 
as it is of course quite erroneous between 0° and 60°. But it is really of compara¬ 
tively little use to tabulate the mean specific heat. The quantity most often 
required is the total heat h from 0° to t. If we adopt as unit the specific heat of 
water at 20° C., the total heat from 0° to 60° is 60'020. The value of the total heat 
h above 60° is then represented by the formula, 
(Above 60°) h = + '220 + '9944* + -000,02$® + ‘000,000,3$ 3 . . (2), 
which differs from Regnault’s formula only in the first two terms, and is deduced 
from the formula (l) for the specific heat at t by integration, and addition of a 
suitable constant to make the value right at 60° C. 
To find the mean specific heat between any two arbitrarily selected temperatures, 
which is often required in reducing calorimetric observations, the simplest method of 
procedure is usually to take the difference between the values of h corresponding to the 
integral values nearest to the extremes of the range, and divide by the whole number 
of degrees between the values taken. This will generally give a result which is 
accurate to 1 in 10,000. If the range is less than 10° the order of accuracy will 
be proportionately less; but this is immaterial, as the same will probably be true of 
the observations themselves with which the comparison is required. 
The work of Pfaundler and Platter, of Hirn, of Jamin and Am a fry, and of 
many other experimentalists who succeeded Regnault, appeared to indicate much 
larger rates of increase than he had found; but there can be little doubt that the 
discrepancies of their results, which often exceeded 5 per cent., were due to lack of 
appreciation of the difficulties of the problem. Before the time of Rowland’s 
experiments in 1879, sufficient attention had not been paid to the thermometry, and 
the results were of comparatively little value. 
(42.) The Work of Rowland. 
It is unnecessary to give any description or criticism of Rowland’s work, which is 
generally recognised as being the most accurate determination of the mechanical 
equivalent of the thermal unit at ordinary temperatures. Rowland himself con¬ 
sidered that his results were probably correct to at least 1 in 500, and that the 
greatest uncertainty lay in the comparison of the scale of his mercury-thermometers 
