14 
PROF. H. L. CALLENDAR ON THE VARIATION OF THE SPECIFIC 
from 0° C. to 80° C. than that assigned by the continuous-electric method, and that 
the error probably lay with the latter. It is usual to take the curves for the actual 
specific heat in making these comparisons, but these do not represent the observations 
themselves, except in the case of the continuous-electric method, and little can fairly 
be deduced from such a comparison (though Dieterici, Ludin, Bouseield, and many 
others have adopted this method), owing to the great uncertainty involved in 
deducing the actual from the mean specific heat. There is no similar uncertainty in 
deducing the mean from the actual specific heat, so that the method adopted in fig. 2 
is the more appropriate. Since the evidence for the slow rate of increase of the 
specific heat between 60° C. and 100° C., in the continuous-electric method, rested 
chiefly on half-a-dozen observations taken under conditions of exceptional difficulty, 
it appeared desirable to confirm them, if possible, over this range by an entirely 
different method, at least equal in accuracy. 
Continuous-Mixture Meth od. 
If two steady currents of fluid at different temperatures are passed through an 
arrangement of concentric tubes called a “heat exchanger,” it is clear that, neglecting 
external heat-loss or gain, the loss of total heat by the hot current will be equal to 
the gain of total heat by the cold current. By measuring the currents and the 
temperatures of inflow and outflow, we have all the data required for determining 
the ratio of the mean specific heats over tlie respective ranges. This method does 
not appear to have been applied to any extent in accurate calorimetry, on account of 
the experimental difficulties involved in regulating and measuring the currents and 
the temperatures simultaneously to a sufficient order of accuracy. As applied to the 
variation of the specific heat of a single fluid, the method permits of a most important 
simplification which does not appear to have been hitherto noticed. In place of 
employing two separate currents, each of which must be measured and regulated to 
the limit of accuracy, the same current is passed twice through the heat exchanger, 
first as a hot current, and then, after suitable cooling, as a cold current, or vice versa. 
If there is no leakage the ratio of the currents is always one of equality, and a 
comparatively rough determination of the absolute value of the current suffices for 
the application of small corrections. The experimental problem is reduced to the 
regulation and measurement of the temperatures, which, taken by itself, is compara¬ 
tively easy. The method possesses the advantage, common to all continuous-flow 
methods, that a knowledge of the water equivalent of the calorimeter and of its 
variation with temperature is not required provided that the conditions are fairly 
steady. There is no uncertainty of lieat-loss in transference, or by evaporation, as 
with an open calorimeter. It is also easy, by a suitable arrangement of the flow- 
tubes, to reduce the external heat-loss almost to a vanishing quantity without the 
employment of vacuum-jackets or elaborate precautions in lagging. The method 
