atus per
unit of heat added. This can be determined by a direct calibration, by
inserting a known quantity of water at a known temperature and observing
the contraction, or weighing the mercury drawn into the apparatus. In
order to be independent of the accuracy of the thermometer employed for
observing the initial temperature of the water introduced, it has been
usual to employ water at 100 deg. C., adopting as unit of heat the "mean
calorie," which is one-hundredth part of the heat given up by one gramme
of water in cooling from 100 deg. to 0 deg. C. The weight of mercury
corresponding to the mean calorie has been determined with considerable
care by a number of observers well skilled in the use of the instrument.
The following are some of their results:--Bunsen, 15.41 mgm.; Velten,
15.47 mgm.; Zakrevski, 15.57 mgm.; Staub, 15.26 mgm. The explanation of
these discrepancies in the fundamental constant is not at all clear, but
they may be taken as an illustration of the difficulties of manipulation
attending the use of this instrument, to which reference has already
been made. It is not possible to deduce a more satisfactory value from
the latent heat and the change of density, because these constants are
very difficult to determine. The following are some of the values
deduced by well-known experimentalists for the latent heat of
fusion:--Regnault, 79.06 to 79.24 calories, corrected by Person to
79.43; Person, 79.99 calories; Hess, 80.34 calories; Bunsen, 80.025
calories. Regnault, Person and Hess employed the method of mixture which
is probably the most accurate for the purpose. Person and Hess avoided
the error of water sticking to the ice by using dry ice at various
temperatures below 0 deg. C., and determining the specific heat of ice
as well as the latent heat of fusion. These discrepancies might, no
doubt, be partly explained by differences in the units employed, which
are somewhat uncertain, as the specific heat of water changes rapidly in
the neighbourhood of 0 deg. C; but making all due allowance for this, it
remains evident that the method of ice-calorimetry, in spite of its
theoretical simplicity, presents grave difficulties in its practical
application.

One of the chief difficulties in the practical use of the Bunsen
calorimeter is the continued and often irregular movement of the
mercury column due to slight differences of temperature, or pressure
between the ice in the calorimeter and the ice bat