COHESION OF ALUM PARTICLES. 41 



those particles together we have many better 

 plans than that, and I will show yon one that 

 will do very well for juvenile experiments. 

 There is some alum crystallised very beautifully 

 by nature (for all things are far more beautiful 

 in their natural than their artificial form), and 

 here I have some of the same alum broken into 

 fine powder. In it I have destroyed that force 

 of which I have placed the name on this board 

 COHESION, or the attraction exerted between 

 the particles of bodies to hold them together. 

 Now I am going to show you that if we take this 

 powdered alum and some hot water, and mix 

 them together, I shall dissolve the alum all 

 the particles will be separated by the water far 

 more completely than they are here in the 

 powder ; but then, being in the water, they will 

 have the opportunity as it cools (for that is the 

 condition which favours their coalescence) of 

 uniting together again and forming one mass.( 7 ) 

 Now, having brought the alum into solution, 

 I will pour it into this glass basin, and you will, 

 to-morrow, find that those particles of alum 

 which I have put into the water, and so separated 

 that they are no longer solid, will, as the water 

 cools, come together and cohere, and by to-mor- 



