COHESION 35 



a wonderful manner they open out to us the internal arrange- 

 ment of the particles of this calcareous spar by the 'force 

 of cohesion. 



And now I will show 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 polarized light, and I will 

 first of all show 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 show 

 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 par- 

 ticle. Whether you have this mechanical power of strain- 

 ing, or whether we take other means, we get the same 

 result; and, indeed, I will show 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 polarized 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 altera- 

 tion 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. 



