Page:On the various forces of nature and their relations to each other.djvu/74

70 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.