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.
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.
Fig. 257.
371. Attraction and Repulsion in Electricity.—One of the most common effects of electricity is attraction. If we rub a tube or rod of glass with woolen or silk it will attract light articles, such as cotton, feathers, lint, etc., so that they will adhere to it. But repulsion is also an effect of electricity under certain circumstances. In order that the explanation of these two opposite effects may be clear to you, I will detail some of the experiments which exhibit both. Suppose that we have a pith ball, A, Fig. 257, suspended by a silk thread, B, from a stand, C. I must premise that silk does not readily let electricity pass over it, or is a non-conductor, and therefore any electricity communicated to the pith ball will remain there unless something be brought in contact with it or very near it. If now you rub a glass tube, thus exciting electricity upon it, and then bring it near the ball, it will attract the ball to it, and then in a moment repel it, so that it will stand off from the tube and retreat from it if you follow the ball with the tube. Why is this? It is supposed that there is a subtile fluid on the electrified glass, some of which passes to the ball as it touches the glass, so that the ball and the glass are in a similar condition. But the particles of the fluid repel each other; and this is the reason that the ball is repelled from the glass as soon as it becomes charged with a part of the electricity of the glass. For the same reason if two pith balls hanging from a standard become electrified from a glass tube or rod they will repel each other, for they are in the same electrical condition.