ch is often recommended, diminishes the loss of heat considerably,
but renders it very uncertain and variable, and should never be used
in work of precision. The bad conductors take so long to reach a
steady state that the rate of loss of heat at any moment depends on
the past history more than on the temperature of the calorimeter at
the moment. A more serious objection to the use of lagging of this
kind is the danger of its absorbing moisture. The least trace of damp
in the lagging, or of moisture condensed on the surface of the
calorimeter, may produce serious loss of heat by evaporation. This is
another objection to Rumford's method of cooling the calorimeter below
the surrounding temperature before starting. Among minor difficulties
of the method may be mentioned the uncertainty of the thermal capacity
of the calorimeter and stirrer, and of the immersed portion of the
thermometer. This is generally calculated by assuming values for the
specific heats of the materials obtained by experiment between 100
deg. C. and 20 deg. C. Since the specific heats of most metals
increase rapidly with rise of temperature, the values so obtained are
generally too high. It is best to make this correction as small as
possible by using a large calorimeter, so that the mass of water is
large in proportion to that of metal. Analogous difficulties arise in
the application of other calorimetric methods. The accuracy of the
work in each case depends principally on the skill and ingenuity of
the experimentalist in devising methods of eliminating the various
sources of error. The form of apparatus usually adopted for the
method of mixtures is that of Regnault with slight modifications, and
figures and descriptions are given in all the text-books. Among
special methods which have been subsequently developed there are two
which deserve mention as differing in principle from the common type.
These are (1) the constant temperature method, (2) the continuous flow
method.

[Illustration: FIG. 1.]

The _constant temperature method of mixtures_ was proposed by N.
Hesehus (_Jour. Phys._, 1888, vii. p. 489). Cold water at a known
temperature is added to the calorimeter, immediately after dropping in
the heated substance, at such a rate as to keep the temperature of the
calorimeter constant, thus eliminating the corrections for the water
equivalent of the calorimeter and the extern