The experiments fit in fairly well with the formula of Van der Waals,
but considerable discrepancies occur when we extend its limits,
particularly when the pressures throughout a rather wider interval are
considered; so that other and rather more complex formulas, on which
there is no advantage in dwelling, have been proposed, and, in certain
cases, better represent the facts.
But the most remarkable result of M. Van der Waals' calculations is
the discovery of corresponding states. For a long time physicists
spoke of bodies taken in a comparable state. Dalton, for example,
pointed out that liquids have vapour-pressures equal to the
temperatures equally distant from their boiling-point; but that if, in
this particular property, liquids were comparable under these
conditions of temperature, as regards other properties the parallelism
was no longer to be verified. No general rule was found until M. Van
der Waals first enunciated a primary law, viz., that if the pressure,
the volume, and the temperature are estimated by taking as units the
critical quantities, the constants special to each body disappear in
the characteristic equation, which thus becomes the same for all
fluids.
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