It is well known that a beam, or pencil, of light, as emitted from the sun, is _not_ a simple body, but is capable of being divided into seven prismatic colors. The image which is formed by the refraction of the pencil, by means of a prism, is called a _Spectrum_, and clearly exhibits the compound nature of light. The refracted rays of the Spectrum may be collected and made to constitute a pencil of light again, which will be white, or colorless as before. If this prismatic Spectrum be examined closely, it will be found that the different colored rays differ very much in their _heating_, _illuminating_, and _chemical_ powers. Dr. Herschell, and other experimenters, have found that the _orange_ rays possess a greater illuminating power than the red; and the _yellow_ more than the orange: but the _maximums_ of illumination lies in the _brightest yellow_ or _palest green_. There is also a very sensible difference in the _heating_ power of these colored rays. By passing the bulb of a delicate air thermometer through the different colored rays, it indicates the greatest heat in the _red_ rays; next in the _green_, and so on diminishing to the _violet_. But the maximum of heat has been ascertained to be immediately _beyond_ the red rays, and of course _out_ of the Spectrum, in an _unilluminated_ spot: thus indicating that there are _invisible_ rays possessing a greater heating power than any of the seven colored rays. These are called _calorific rays_. By the experiments of Ritter and Wallaston it is now satisfactorily ascertained that there are also _chemical rays which excite neither heat nor light_, and lie on the _other side_ of the Spectrum from the invisible calorific rays, just without the violet. It is true, the chemical effect can be distinguished even to the green rays, but this seems to be by _diffusion_, or a species of sympathy. The sensible chemical power is exerted just without the violet rays. This fact is established more clearly by Berard. He concentrated, by a lens, all the portion of the Spectrum from the green to the red rays, and made them act on muriate of silver _two hours_ without effect. He then concentrated all the portion of the Spectrum from the green to the violet rays, and made them act on muriate of silver, and _they blackened it in less than six minutes_. Thus, evidently, are detected very different properties in the different portions of the prismatic Spectrum. Instead, therefore, of light being a “simple substance,” and “unmixed” it is found to be decidedly _compound_. It is capable of being divided into seven differently colored rays, and these rays, according to their natural properties, into three classes: the _illuminating_ rays, _calorific_ rays, and _chemical_ rays.]