368. =Heat and Light.=--We have not yet finished our dissection of the beam of light, begun in § 360. In the beam of light which is separated into the seven colors there is heat also; and in the separation it is found, as represented in Fig. 256, that the rays of heat are most abundant just beyond the red rays, while they are very sparing indeed at the other end of the spectrum. The greatest degree of light is at the boundary between the orange and the yellow rays. [Illustration: Fig. 256.] 369. =Chemistry of Light and the Daguerreotype.=--There is a chemical power also in light, producing every where, quietly but thoroughly, important effects. The chemical rays are most abundant at the end of the spectrum opposite to that where the heat-rays abound. It is these which do the work in Daguerreotyping. In this art light has been said to be the painter; but this is not strictly true. Light makes the image of the object, just as it does in the camera obscura and in the eye, but it has no power to fasten that image upon the metallic plate. This is done by the chemical rays, which, like the rays of heat, go along with the light. Without going into particulars, which will be given in Part Second, the process of Daguerreotyping is simply this: A metallic plate is so prepared that the chemical rays of light shall act upon it sensibly. Then the object to be taken--a person or any thing else--being before the instrument, a slip of ground glass is inserted, and when the operator gets the lens so adjusted that a good image of the object is seen on the glass he takes this out and puts in its place the metallic plate. Rays of light coming from the object make the image, and the chemical rays bound up with the light act upon the plate so as to fix the image there. CHAPTER XV. ELECTRICITY. 370. =Origin of the Term.=--The ancients observed that when certain substances were rubbed together singular phenomena were produced. One of these substances was amber, and as the Greek name for this is ηλεκτρον, the power which is thus excited into action has been called electricity. [Illustration: Fig. 257.]