The results which have been obtained in the examination of radio-activity, of kathode rays, of spectra at different temperatures, and of phenomena allied to these, bring again into prominence the ancient problem of the structure of what we call matter. Is matter fundamentally homogeneous or heterogeneous? Chemistry studies the relations between the changes of composition and the changes of properties which happen simultaneously in material systems. The burning fire of wood, coal, or gas; the preparation of food to excite and to satisfy the appetite; the change of minerals into the iron, steel, copper, brass, lead, tin, lighting burning and lubricating oils, dye-stuffs and drugs of commerce; the change of the skins, wool, and hair of animals, and of the seeds and fibres of plants, into clothing for human beings; the manufacture from rags, grass, or wood of a material fitted to receive and to preserve the symbols of human hopes, fears, aspirations, love and hate, pity and aversion; the strange and most delicate processes which, happening without cessation, in plants and animals and men, maintain that balanced equilibrium which we call life; and, when the silver cord is being loosed and the bowl broken at the cistern, the awful changes which herald the approach of death; not only the growing grass in midsummer meadows, not only the coming of autumn "in dyed garments, travelling in the glory of his apparel," but also the opening buds, the pleasant scents, the tender colours which stir our hearts in "the spring time, the only pretty ring time, when birds do sing, ding-a--dong-ding": these, and a thousand other changes have all their aspects which it is the business of the chemist to investigate. Confronted with so vast a multitude of never-ceasing changes, and bidden to find order there, if he can--bidden, rather compelled by that imperious command which forces the human mind to seek unity in variety, and, if need be, to create a cosmos from a chaos; no wonder that the early chemists jumped at the notion that there must be, that there is, some _One Thing_, some _Universal Essence_, which binds into an orderly whole the perplexing phenomena of nature, some _Water of Paradise_ which is for the healing of all disorder, some "Well at the World's End," a draught whereof shall bring peace and calm security.