Page:On the various forces of nature and their relations to each other.djvu/75

Rh Whether you have this mechanical power of straining, or whether we take other means, we get the same result; and, indeed, I will shew you by another experiment that if we heat the glass in one part, it will alter its internal structure, and produce a similar effect. Here is a piece of common glass, and if I insert this in the path of the polarised ray, I believe it will do nothing. There is the common glass [introducing it]—no light passes through—the screen remains quite dark; but I am going to warm this glass in the lamp, and you know yourselves that when you pour warm water upon glass you put a strain upon it sufficient to break it sometimes—something like there was in the case of the Prince Rupert's drops. [The glass was warmed in the spirit-lamp, and again placed across the ray of light.] Now you see how beautifully the light goes through those parts which are hot, making dark and light lines just as the crystal did, and all because of the alteration I have effected in its internal condition; for these dark and light parts are a proof of the presence of forces acting and dragging in different directions within the solid mass.