Page:Outlines of Physical Chemistry - 1899.djvu/63

 AmagaVs results for the gases, nitrogen, carbon dioxide, methane, and ethylene are graphically represented. In the case of hydrogen we only know the rising part of the curve ; from the first, hydrogen is too little compressible. Begnault has termed it a ' more than perfect gas ' (' gaz plus que parfait ').

Liquefaction of Gases, The Critical State

When a gas is submitted to an increasing pressure at constant temperature (that is, the operation must be so carried out that this condition is fulfilled), we can observe two different cases :

1. Either, the gas diminishes in volume, at first more rapidly than Boyle's law demands, then regularly (the product pv remaining almost constant), and finally the volume diminishes more and more slowly. The gas then behaves almost like a liquid, and the product p v rises very rapidly.

Gases of this class have for a long time been considered as permanent, or non-liquefiable.

2. Or, the gas diminishes in volume more rapidly than Boyle's law requires (the product pv decreases) and when the pressure has attained a certain value, the compressed matter is no longer homogeneous, but sepa- rates into two layers, one gaseous and the other liquid. A further decrease of volume does not correspond to a further increase of pressure, but simply to an increase of the liquid layer at the expense of the gaseous layer. A moment arrives when the gas has disappeared, and the whole available space is occupied by liquid; from that moment the laws of the compression of liquids become applicable — even a very large increase of pressure produces only a very small diminution of volume, that is, the product p v increases almost proportionately with the pressure.

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1 Outlines of General Chemistry, translated by Walker.

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