56 COHESION. 



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 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. Plere, 

 then, we have our ray of polarised 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 ho wit makes the light come 

 through some parts and leaves it dark in others, 

 and all because we weaken the force of cohesion 



