of taking to pieces led to the discovery of the way to put together; and vast numbers of compounds, some of them previously known only as products of the living economy, have thus been artificially constructed.  Chemical work, at the present day, is, to a large extent, synthetic or creative—­that is to say, the chemist determines, theoretically, that certain non-existent compounds ought to be producible, and he proceeds to produce them.

It is largely because the chemical theory and practice of our epoch have passed into this deductive and synthetic stage, that they are entitled to the name of the ‘New Chemistry’ which they commonly receive.  But this new chemistry has grown up by the help of hypotheses, such as those of Dalton and of Avogadro, and that singular conception of ‘bonds’ invented to colligate the facts of ‘valency’ or ‘atomicity,’ the first of which took some time to make its way; while the second fell into oblivion, for many years after it was propounded, for lack of empirical justification.  As for the third, it may be doubted if anyone regards it as more than a temporary contrivance.

But some of these hypotheses have done yet further service.  Combining them with the mechanical theory of heat and the doctrine of the conservation of energy, which are also products of our time, physicists have arrived at an entirely new conception of the nature of gaseous bodies and of the relation of the physico-chemical units of matter to the different forms of energy.  The conduct of gases under varying pressure and temperature, their diffusibility, their relation to radiant heat and to light, the evolution of heat when bodies combine, the absorption of heat when they are dissociated, and a host of other molecular phenomena, have been shown to be deducible from the dynamical and statical principles which apply to molar motion and rest; and the tendency of physico-chemical science is clearly towards the reduction of the problems of the world of the infinitely little, as it already has reduced those of the infinitely great world, to questions of mechanics.[H]

In the meanwhile, the primitive atomic theory, which has served as the scaffolding for the edifice of modern physics and chemistry, has been quietly dismissed.  I cannot discover that any contemporary physicist or chemist believes in the real indivisibility of atoms, or in an interatomic matterless vacuum.  ‘Atoms’ appear to be used as mere names for physico-chemical units which have not yet been subdivided, and ‘molecules’ for physico-chemical units which are aggregates of the former.  And these individualised particles are supposed to move in an endless ocean of a vastly more subtle matter—­the ether.  If this ether is a continuous substance, therefore, we have got back from the hypothesis of Dalton to that of Descartes.  But there is much reason to believe that science is going to make a still further journey, and, in form, if not altogether in substance, to return to the point of view of Aristotle.