100 deg. C.,
very nearly. The thermal capacity of the scale-pan, &c., can be
determined by a separate experiment, or, still better, eliminated by the
differential method of counterpoising with an exactly similar
arrangement on the other arm of the balance. The method requires very
delicate weighing, as one calorie corresponds to less than two
milligrammes of steam condensed; but the successful application of the
method to the very difficult problem of measuring the specific heat of a
gas at constant volume, shows that these and other difficulties have
been very skilfully overcome. The application of the method appears to
be practically limited to the measurements of specific heat between the
atmospheric temperature and 100 deg. C. The results depend on the value
assumed for the latent heat of steam, which Joly takes as 536.7
calories, following Regnault. Joly has himself determined the mean
specific heat of water between 12 deg. and 100 deg. C. by this method,
in terms of the latent heat of steam as above given, and finds the
result .9952. Assuming that the mean specific heat of water between 12
deg. and 100 deg. is really 1.0011 in terms of the calorie at 20 deg. C.
(see table, p. 66), the value of the latent heat of steam at 100 deg.
C., as determined by Joly, would be 540.2 in terms of the same unit. The
calorie employed by Regnault is to some extent uncertain, but the
difference is hardly beyond the probable errors of experiment, since it
appears from the results of recent experiments that Regnault made an
error of the same order in his determination of the specific heat of
water at 100 deg. C.

S 6. _Energy Methods._--The third general method of calorimetry, that
based on the transformation of some other kind of energy into the form
of heat, rests on the general principle of the conservation of energy,
and on the experimental fact that all other forms of energy are readily
and completely convertible into the form of heat. It is therefore often
possible to measure quantities of heat indirectly, by measuring the
energy in some other form and then converting it into heat. In addition
to its great theoretical interest, this method possesses the advantage
of being frequently the most accurate in practical application, since
energy can be more accurately measured in other forms than in that of
heat. The two most important varieties of the method are (_a_)
mechanical, and (_b_) electrical. These methods have reached their