In the case of coloured bodies, too, the particles forming their surfaces reflect white light like those of all other bodies, so that these particles cannot produce colour on the same principles as those of thin plates. In many of those cases of colour which seem to depend upon the minuteness of the particles of the body, the reflection of white light may nevertheless be observed, but this will be found to arise from a thin transparent film, behind which the colorific particles are placed. Whatever answer may be given to these objections, we think it will be admitted by those who have studied the subject most profoundly, that a satisfactory theory of the colours of natural bodies is still a _desideratum_ in science. How far we may be able to approach to it in the present state of optics the reader will judge from the following views. Colours may be arranged into seven classes, each of which depends upon different principles. 1. Transparent coloured fluids—transparent coloured gems—transparent coloured glasses—coloured powders—and the colours of the leaves and flowers of plants. 2. Oxidations on metals—colours of Labrador feldspar—colours of precious and hydrophanous opal, and other opalescences—the colours of the feathers of birds, of the wings of insects, and of the scales of fishes. 3. Superficial colours, as those of mother-of-pearl and striated surfaces. 4. Opalescences and colours in composite crystals having double refraction. 5. Colours from the absorption of common and polarized light by doubly refracting crystals. 6. Colours at the surfaces of media of different dispersive powers. 7. Colours at the surface of media in which the reflecting forces extend to different distances, or follow different laws. The first two of these classes are the most important. The Newtonian theory appears to be strictly applicable to the phenomena of the _second_ class; but those of the first class cannot, we conceive, be referred to the same cause. * * * * * The rays of solar light possess several remarkable physical properties: They heat—they illuminate—they promote chymical combination—they effect chymical decompositions—they impart magnetism to steel—they alter the colours of bodies—they communicate to plants and flowers their peculiar colours, and are in many cases necessary to the development of their characteristic qualities. It is impossible to admit for a moment that these varied effects are produced by a mere mechanical action, or that they arise from the agitation of the particles of bodies by the vibration of the ether which is considered to be the cause of light. Whatever be the difficulties which attach to the theory which supposes light to consist of material particles, we are compelled, by its properties, to admit that light acts as if it were material, and that it enters into combinations with bodies, in order to produce the effects which we have enumerated.