26 FARADAY 



attraction of their particles, and if I press these two sepa- 

 rate pieces close together, so as to bring their particles 

 within the sphere of' attraction, you will see how soon they 

 become one. I have merely to give them a good squeeze, 

 and draw the upper piece slightly round at the same time, 

 and here they are as one, and all the bending and twisting I 

 can give them will not separate them again; I have joined 

 the lead together, not with solder, but simply by means of 

 the attraction of the particles. 



This, however, is not the best way of bringing those par- 

 ticles together; we have many better plans than that; and 

 I will show you one that will do very well for juvenile 

 experiments. There is some alum crystallized very beauti- 

 fully 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 favors their coalescence) 

 of uniting together again and forming one mass ( e ). 



Now, having brought the alum into solution, I will pour 

 it into this glass basin, and you will, to-morrow, find that 

 these 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-morrow morning 

 we shall have a great deal of the alum crystallized out that 

 is to say, come back to the solid form. [The lecturer here 

 poured a little of the hot solution of alum into the glass 

 dish, and when the latter had thus been made warm, the 

 remainder of the solution was added.] I am now doing that 



Crystallisation of alum. The solution must be saturated that is, it 

 must contain as much alum as can possibly be dissolved. In making the 

 solution, it is best to add powdered alum to hot water as long as it dis- 

 solves; and when no more is taken up, allow the solution to stand a few 

 minutes, and then pour it off from the dirt and undissolved alum. 



