II

$v$ is a known number. But this is only in particular cases. In considering generally the properties of the gas and how it will behave, he has to have in his mind the general form of the whole curve $ABC$ and its general properties. In other words the really fundamental idea is that of the pair of variables

satisfying the relation $pv=1$. This example illustrates how the idea of variables is fundamental,

both in the applications as well as in the theory of mathematics.