t of temperature. A calorimeter is any piece of
apparatus in which heat is measured. This distinction of meaning is
purely a matter of convention, but it is very rigidly observed.
Quantities of heat may be measured indirectly in a variety of ways in
terms of the different effects of heat on material substances. The most
important of these effects are (_a_) rise of temperature, (_b_) change
of state, (_c_) transformation of energy.

S 1. The rise of temperature of a body, when heat is imparted to it, is
found to be in general nearly proportional to the quantity of heat
added. The _thermal capacity_ of a body is measured by the quantity of
heat required to raise its temperature one degree, and is necessarily
proportional to the mass of the body for bodies of the same substance
under similar conditions. The _specific heat_ of a substance is
sometimes defined as the thermal capacity of unit mass, but more often
as the ratio of the thermal capacity of unit mass of the substance to
that of unit mass of water at some standard temperature. The two
definitions are identical, provided that the thermal capacity of unit
mass of water, at a standard temperature, is taken as the unit of heat.
But the specific heat of water is often stated in terms of other units.
In any case it is necessary to specify the temperature, and sometimes
also the pressure, since the specific heat of a substance generally
depends to some extent on the external conditions. The methods of
measurement, founded on rise of temperature, may be classed as
_thermometric methods_, since they depend on the observation of change
of temperature with a thermometer. The most familiar of these are the
method of mixture and the method of cooling.

S 2. The _Method of Mixture_ consists in imparting the quantity of
heat to be measured to a known mass of water, or some other standard
substance, contained in a vessel or calorimeter of known thermal
capacity, and in observing the rise of temperature produced, from
which data the quantity of heat may be found as explained in all
elementary text-books. This method is the most generally convenient
and most readily applicable of calorimetric methods, but it is not
always the most accurate, for various reasons. Some heat is generally
lost in transferring the heated body to the calorimeter; this loss may
be minimized by performing the transference rapidly, but it cannot be
accurately calculated or eliminated. S