Page:Outlines of Physical Chemistry - 1899.djvu/64

 44 OUTLINES OF PHYSICAL CHEMISTRY

This is the way in which liquefiable gases behave.

All gases are liquefiable, but not all at the ordinary temperature nor even at 0°C. Hydrogen and some other gases (0 2, N 2 , CO, CH 4 , NO) were for a long time considered as permanent — it was found to be impossible to condense them even with extremely high pressures.

The explanation of these phenomena, already foreseen by Cagniard de la Tour, was given by Andrews in 1869. For every gas there exists a critical temperature, above which liquefaction is impossible, and at which liquefaction takes place under the influence of a certain definite pressure and without change of volume.

We find here a transition — a kind of confusion — from the gaseous to the liquid state ; and we can see the pos- sibility of passing from the one state to the other without the occurrence of any discontinuity in the condition of the substance, that is, without any separation into two layers.

(a) Starting with a gas, we can bring this transition about as follows : Heat the gas to a temperature higher than its critical temperature, then compress it to a pressure higher than its critical pressure, and after that cool it to the critical temperature. Then liquefaction takes place without the formation of two layers, the liquid formed occupies the whole space previously filled by the gas.

(b) Starting with a liquid, we first submit it to a pressure higher than the critical pressure, then raise its temperature above the critical temperature. There will be no separation into two layers, but the whole of the sub- stance will assume the gaseous condition. Finally the gas is allowed to expand (furnishing the necessary heat, so that no partial condensation occurs), and the end in view is realised, namely, an absolutely gradual passage from the liquid to the gaseous state without any discontinuity in the homogeneous condition of the substance.

Experimental Work. 1 — Before the publication of

1 The first liquefaction of a gas (chlorine) was performed by Northmore in 1806.— Tr.

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