l
nature of the quantity measured.

Magnitudes to which we attribute like dimensions may be qualitatively
irreducible one to the other. Thus the different forms of energy are
measured by the same unit, and yet it seems that some of them, such as
kinetic energy, really depend on time; while for others, such as
potential energy, the dependency established by the system of
measurement seems somewhat fictitious.

The numerical value of a quantity of energy of any nature should, in
the system C.G.S., be expressed in terms of the unit called the erg;
but, as a matter of fact, when we wish to compare and measure
different quantities of energy of varying forms, such as electrical,
chemical, and other quantities, etc., we nearly always employ a method
by which all these energies are finally transformed and used to heat
the water of a calorimeter. It is therefore very important to study
well the calorific phenomenon chosen as the unit of heat, and to
determine with precision its mechanical equivalent, that is to say,
the number of ergs necessary to produce this unit. This is a number
which, on the principle of equivalence, depends neither on the method
employed, nor the time, nor any other external circumstance.

As the result of the brilliant researches of Rowland and of Mr
Griffiths on the variations of the specific heat of water, physicists
have decided to take as calorific standard the quantity of heat
necessary to raise a gramme of water from 15 deg. to 16 deg. C., the
temperature being measured by the scale of the hydrogen thermometer of
the International Bureau.

On the other hand, new determinations of the mechanical equivalent,
among which it is right to mention that of Mr. Ames, and a full
discussion as to the best results, have led to the adoption of the
number 4.187 to represent the number of ergs capable of producing the
unit of heat.

In practice, the measurement of a quantity of heat is very often
effected by means of the ice calorimeter, the use of which is
particularly simple and convenient. There is, therefore, a very
special interest in knowing exactly the melting-point of ice. M.
Leduc, who for several years has measured a great number of physical
constants with minute precautions and a remarkable sense of precision,
concludes, after a close discussion of the various results obtained,
that this heat is equal to 79.1 calories. An error of almost a calorie
had been committed by several renowned experiment