iven: (1) the thermometric unit, or
the thermal capacity of unit mass of a standard substance under given
conditions of temperature and pressure on the scale of a standard
thermometer. (2) The latent-heat unit, or the quantity of heat required
to melt or vaporize unit mass of a standard substance under given
conditions. This unit has the advantage of being independent of
thermometry, but the applicability of these methods is limited to
special cases, and the relation of the units to other units is difficult
to determine. (3) The absolute or mechanical unit, the quantity of heat
equivalent to a given quantity of mechanical or electrical energy. This
can be very accurately realized, but is not so convenient as (1) for
ordinary purposes.

In any case it is necessary to define a thermometric unit of class
(1). The standard substance must be a liquid. Water is always
selected, although some less volatile liquid, such as aniline or
mercury, would possess many advantages. With regard to the scale of
temperature, there is very general agreement that the absolute scale
as realized by the hydrogen or helium thermometer should be adopted as
the ultimate standard of reference. But as the hydrogen thermometer is
not directly available for the majority of experiments, it is
necessary to use a secondary standard for the practical definition of
the unit. The electrical resistance thermometer of platinum presents
very great advantages for this purpose over the mercury thermometer in
point of reproducibility, accuracy and adaptability to the practical
conditions of experiment. The conditions of use of a mercury
thermometer in a calorimetric experiment are necessarily different
from those under which its corrections are determined, and this
difference must inevitably give rise to constant errors in practical
work. The primary consideration in the definition of a unit is to
select that method which permits the highest order of accuracy in
comparison and verification. For this reason the definition of the
thermal unit will in the end probably be referred to a scale of
temperature defined in terms of a standard platinum thermometer.

There is more diversity of opinion with regard to the question of the
standard temperature. Many authors, adopting Regnault's formula, have
selected 0 deg. C. as the standard temperature, but this cannot be
practically realized in the case of water, and his form