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.