In awarding to Maxwell the Rumford medal in 1860, Major-General Sabine, vice-president of the Royal Society, after explaining the theory of colour vision and the possible method of verifying it, said: “Professor Maxwell has subjected the theory to this verification, and thereby raised the composition of colours to the rank of a branch of mathematical physics,” and he continues: “The researches for which the Rumford medal is awarded lead to the remarkable result that to a very near degree of approximation all the colours of the spectrum, and therefore all colours in nature which are only mixtures of these, can be perfectly imitated by mixtures of three actually attainable colours, which are the red, green and blue belonging respectively to three particular parts of the spectrum.” It should be noticed in concluding our remarks on this part of Maxwell’s work that his results are purely physical. They are not inconsistent with the physiological part of Young’s theory, viz., that there are three primary sensations of colour which can be transmitted to the brain, and that the colour of any object depends on the relative proportions in which these sensations are excited, but they do not prove that theory. Any physiological theory which can be accepted as true must explain Maxwell’s observations, and Young’s theory does this; but it is, of course, possible that other theories may explain them equally well, and be more in accordance with physiological observations than Young’s. Maxwell has given us the physical facts which have to be explained; it is for the physiologists to do the rest. CHAPTER VIII. SCIENTIFIC WORK--MOLECULAR THEORY. Maxwell in his article “Atom,” in the ninth edition of the _Encyclopædia Britannica_, has given some account of Modern Molecular Science, and in particular of the molecular theory of gases. Of this science, Clausius and Maxwell are the founders, though to their names it has recently been shown that a third, that of Waterston, must be added. In the present chapter it is intended to give an outline of Maxwell’s contributions to molecular science, and to explain the advances due to him. The doctrine that bodies are composed of small particles in rapid motion is very ancient. Democritus was its founder, Lucretius--de Rerum Naturâ--explained its principles. The atoms do not fill space; there is void between. “Quapropter locus est intactus inane vacansque, Quod si non esset, nullâ ratione moveri Res possent; namque officium quod corporis extat Officere atque obstare, id in omni tempore adesset Omnibus. Haud igitur quicquam procedere posset Principium quoniam cedendi nulla daret res.”