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° 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° from the indications of the hydrogen thermometer
towards the temperature -240° C, and add +0.05° to 1000° 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° C. downward the divergence of such a thermometer from one of
hydrogen is very marked.

The measurement of very high temperatures is not open to the same
theoretical objections as that of very low temperatures; but, from a
practical point of view, it is as difficult to effect with an ordinary