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.