EFFECT OF HEAT OR PRESSURE ON GLASS. 57 



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 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 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 Eupert'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. 



