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Now it happens that if we
make use for the estimation of temperatures, of the phenomena of
dilatation under a constant pressure, or of the increase of pressure
in a constant volume of a gaseous body, we obtain a scale very near
the absolute, which almost coincides with it when the gas possesses
certain qualities which make it nearly what is called a perfect gas.
This most lucky coincidence has decided the choice of the convention
adopted by physicists. They define normal temperature by means of the
variations of pressure in a mass of hydrogen beginning with the
initial pressure of a metre of mercury at 0 deg. C.
M.P. Chappuis, in some very precise experiments conducted with much
method, has proved that at ordinary temperatures the indications of
such a thermometer are so close to the degrees of the theoretical
scale that it is almost impossible to ascertain the value of the
divergences, or even the direction that they take. The divergence
becomes, however, manifest when we work with extreme temperatures. It
results from the useful researches of M.


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