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 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
gas thermometer.
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