t it should constantly vary in the same direction
when the temperature rises, and that it should possess, at any
temperature, a well-marked value. We measure this value by melting ice
and by the vapour of boiling water under normal pressure, and the
successive hundredths of its variation, beginning with the melting
ice, defines the percentage. Thermodynamics, however, has made it
plain that we can set up a thermometric scale without relying upon any
determined property of a real body. Such a scale has an absolute value
independently of the properties of matter. 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. Daniel Berthelot that we must
subtract +0.18 deg. from the indications of the hydrogen thermometer
towards the temperature -240 deg. C, and add +0.05 deg. to 1000 deg.
to equate them with the thermodynamic scale. Of course, the difference
would also become still more noticeable on getting nearer to the
absolute zero; for as hydrogen gets more and more cooled, it gradually
exhibits in a lesser degree the characteristics of a perfect gas.

To study the lower regions which border on that kind of pole of cold
towards which are straining the efforts of the many physicists who
have of late years succeeded in getting a few degrees further forward,
we may turn to a gas still more difficult to liquefy than hydrogen.
Thus, thermometers have been made of helium; and from the temperature
of -260 deg. C. downward the divergence of