e on its
terminals was nearly 30 volts. The rate of stirring adopted was so
slow that the heat generated by it could be neglected. The result
found was 4.191 joules per calorie at 19 deg. C. This agrees very well
with Griffiths considering the difficulty of measuring so small a rise
of temperature at 2 deg. with a mercury thermometer. Admitting that the
electro-chemical equivalent of silver increases with the age of the
solution, a fact subsequently discovered, and that the E.M.F. of the
Clark cell is probably less than 1.4340 volts (the value assumed by
Schuster and Gannon), there is no difficulty in reconciling the result
with that of Rowland.

S 14. _H.L. Callendar and H.T. Barnes_ (_Brit. Assoc. Reports_, 1897
and 1899) adopted an entirely different method of calorimetry, as well
as a different method of electrical measurement. A steady current of
liquid, Q grammes per second, of specific heat, Js joules per degree,
flowing through a fine tube, A B, fig. 9, is heated by a steady
electric current during its passage through the tube, and the
difference of temperature d[theta] between the inflowing and the
outflowing liquid is measured by a single reading with a delicate pair
of differential platinum thermometers at A and B. The difference of
potential E between the ends of the tube, and the electric current C
through it, are measured on an accurately calibrated potentiometer, in
terms of a Clark cell and a standard resistance. If hd[theta] is the
radiation loss in watts we have the equation,

EC = JsQd[theta] + hd[theta] (2).

[Illustration: FIG. 9.]

The advantage of this method is that all the conditions are steady, so
that the observations can be pushed to the limit of accuracy and
sensitiveness of the apparatus. The water equivalent of the
calorimeter is immaterial, since there is no appreciable change of
temperature. The heat-loss can be reduced to a minimum by enclosing
the flow-tube in a hermetically sealed glass vacuum jacket. Stirring
is effected by causing the water to circulate spirally round the bulbs
of the thermometers and the heating conductor as indicated in the
figure. The conditions can be very easily varied through a wide range.
The heat-loss hd[theta] is determined and eliminated by varying the
flow of liquid and the electric current simultaneously, in such a
manner as to secure approximately the same