53 



LECTUEE III. 



COHESION. CHEMICAL AFFINITY. 



WE will first return for a few minutes to one of 

 the experiments made yesterday. You remem- 

 ber what we put together on that occasion 

 powdered alum and warm water ; here is one of 

 the basins then used. Nothing has been done to 

 it since ; but you will find on examining it that 

 it no longer contains any powder, but a multi- 

 tude of beautiful crystals. Here also are the 

 pieces of coke which 1 put into the other basin, 

 they have a fine mass of crystals about them. 

 That other basin I will leave as it is. I will not 

 pour the water from it, because it will show you 

 that the particles of alum have done something 

 more than merely crystallise together. They 

 have pushed the dirty matter from them, laying 

 it around the outside or outer edge of the lower 

 crystals squeezed out as it were by the strong 



