rate work it is essential that the liquid in the
calorimeter should be continuously stirred, and also in the enclosure,
the lid of which must be water-jacketed, and kept at the same steady
temperature as the sides. When all these precautions are taken, the
method loses most of the simplicity which is its chief advantage. It
cannot be satisfactorily applied to the case of solids or powders, and
is much less generally useful than the method of mixture.

S 4. _Method of Fusion._--The methods depending on change of state are
theoretically the simplest, since they do not necessarily involve any
reference to thermometry, and the corrections for external loss of heat
and for the thermal capacity of the containing vessels can be completely
eliminated. They nevertheless present peculiar difficulties and
limitations, which render their practical application more troublesome
and more uncertain than is usually supposed. They depend on the
experimental fact that the quantity of heat required to produce a given
change of state (e.g. to convert one gramme of ice at 0 deg. C. into
water at 0 deg. C., or one gramme of water at 100 deg. C. into steam at
100 deg. C.) is always the same, and that there need be no change of
temperature during the process. The difficulties arise in connexion with
the determination of the quantities of ice melted or steam condensed,
and in measuring the latent heat of fusion or vaporization in terms of
other units for the comparison of observations. The earlier forms of
ice-calorimeter, those of Black, and of Laplace and Lavoisier, were
useless for work of precision, on account of the impossibility of
accurately estimating the quantity of water left adhering to the ice in
each case. This difficulty was overcome by the invention of the Bunsen
calorimeter, in which the quantity of ice melted is measured by
observing the diminution of volume, but the successful employment of
this instrument requires considerable skill in manipulation. The sheath
of ice surrounding the bulb must be sufficiently continuous to prevent
escape of heat, but it must not be so solid as to produce risk of
strain. The ideal condition is difficult to secure. In the practical use
of the instrument it is not necessary to know both the latent heat of
fusion of ice and the change of volume which occurs on melting; it is
sufficient to determine the change of volume per calorie, or the
quantity of mercury which is drawn into the bulb of the appar