28 THOMAS WHITTAKER : Under Mechanics come the Laws of Motion and the Theory of Gravitation. The latter theory was first definitely attained as the result of investigations in the concrete science of Astronomy. This, again, illustrates the relation just re- ferred to. Gravity belongs to General Physics in so far as its theory, once attained, can be stated and worked out with reference to hypothetical masses, and without taking account of the actual masses and distances, empirically ascertained, of particular bodies in the universe. This distinction, in- sisted on by Mr. Spencer, was adumbrated in ancient schemes, Peripatetic or Platonic, by the division of the rational theory of the Sphere from Astronomy regarded as a partially em- pirical science ; though the ancient distinction agreed more nearly with Comte's view in so far as the doctrine of the Sphere was assigned to Mathematics. The divisions of Special Physics are in part determined by the particular senses receptive of the phenomena grouped together. Light, Heat and Sound refer unambiguously to the senses of Sight, Temperature and Hearing. These senses are not, indeed, allowed a share in the scientific ex- planation, which is referred to the so-called "primary qualities of matter," appreciated by the senses of touch and pressure ; but without them the phenomena could not for us have been grouped together at all. Several senses being given, how- ever, combined observations enable us to mark off other groups of phenomena which do not, as such, appear to a particular sense. Metaphor apart, we have no sensations of attraction or repulsion. Hence gravitation could not be directly observed, but had to be inferred from its effects in the form of pressure or motion. Electrical and magnetic phenomena have had to be indirectly appreciated in more various ways. Their common features once known, they could be made the subject of a branch of Special Physics, referred, like the others, to Mechanics or General Physics as fundamental. The reason why Mechanics is thus funda- mental seems to consist essentially in the more permanently numerable and measurable character of the phenomena of perception that are its material. Of Chemistry we may say generally that it deals with the compositions and decompositions of kinds of matter ; whereas molecular Physics deals with states of aggregation of particles conceived as all alike. The complex way, however, in which Chemistry furnishes problems to Physics makes the borders of the two sciences difficult to define. For the perception of the qualitative changes going with changes of composition, it is worthy of note that the senses of taste and smell are
