This equation is extremely interesting, because it now does not contain anything peculiar to one substance, but is applicable to all bodies. It gives rise to the idea that to compare the properties of several substances, liquid or gaseous, it would be rational to take these substances in corresponding states, that is, at temperatures 0T lt and under the pressures irp l9 such that 6 and it have the same value for all the bodies compared. Unfortunately the critical constants are not known for a sufficient number of substances to enable us yet to carry this method very far. We have, however, already seen an application of this principle — the calculation of the critical temperature of a substance in terms of its boiling point (absolute scale).
II. THE LIQUID STATE
The characteristic of the liquid state is that a given mass of liquid has a definite volume but no definite form.
Liquids are but slightly compressible.
The expansion by heat varies according to the nature of the liquid. The behaviour of water on heating is peculiar. From 0° to 4°C. contraction takes place ; from 4° to 50°C. it expands by about 1 per cent, of its volume at 0° ; from 50° to 100°C. it expands still further by about 3 per cent, of the same volume.
The Boiling Point
In order that the correct boiling point of a substance may be determined by distillation, 1 the following points must be observed.
mentally obtained results in fig. 6. — Clausius has modified the equation by introducing a third constant. He has thus succeeded in calculating with very close approximation the characteristics of the critical state. Preference, however, has here been given to Van der WaaWs equation because it is relatively simple, qualitatively satisfac- tory, and the first proposed.
1 The operation is carried out in one or other of the apparatus shown in fig. 8, a and 6. The bulb of the flask should be filled to a third or a half with the substance to be distilled. During the course
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