On the various forces of nature and their relations to each other. Michael Faraday

      Fig. 20.

      Now we will see how calcareous spar acts upon this light,—that stone which split up into rhombs, and of which you are each of you going to take a little piece home. [The mica was removed, and a piece of calc-spar introduced at A.] See how that turns the light round and round, and produces these rings and that black cross (fig. 20). Look at those colours—are they not most beautiful for you and for me?—for I enjoy these things as much as you do. In what a wonderful manner they open out to us the internal arrangement of the particles of this calcareous spar by the force of cohesion.

      And now I will shew you another experiment. Here is that piece of glass which before had no action upon the light. You shall see what it will do when we apply pressure to it. Here, then, we have our ray of polarised light, and I will first of all shew you that the glass has no effect upon it in its ordinary state,—when I place it in the course of the light, the screen still remains dark. Now, Dr. Tyndall will press that bit of glass between three little points, one point against two, so as to bring a strain upon the parts, and you will see what a curious effect that has. [Upon the screen two white dots gradually appeared.] Ah! these points shew the position of the strain—in these parts the force of cohesion is being exerted in a different degree to what it is in the other parts, and hence it allows the light to pass through. How beautiful that is—how it makes the light come through some parts, and leaves it dark in others, and all because we weaken the force of cohesion between particle and particle. 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.