We may think that researches of this kind will only be successful if attention is concentrated, not only on the phenomena of compressibility and dilatation, but also on the calorimetric properties of bodies.

Thermodynamics indeed establishes relations between those properties and other constants, but does not allow everything to be foreseen.
Several physicists have effected very interesting calorimetric measurements, either, like M.Perot, in order to verify Clapeyron's formula regarding the heat of vaporization, or to ascertain the values of specific heats and their variations when the temperature or the pressure happens to change.

M.Mathias has even succeeded in completely determining the specific heats of liquefied gases and of their saturated vapours, as well as the heat of internal and external vaporization.
Sec.2.THE LIQUEFACTION OF GASES, AND THE PROPERTIES OF BODIES AT A LOW TEMPERATURE The scientific advantages of all these researches have been great, and, as nearly always happens, the practical consequences derived from them have also been most important.

It is owing to the more complete knowledge of the general properties of fluids that immense progress has been made these last few years in the methods of liquefying gases.
From a theoretical point of view the new processes of liquefaction can be classed in two categories.

Linde's machine and those resembling it utilize, as is known, expansion without any notable production of external work.