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-morrow morning we shall have a great deal of the alum crystallised 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 which I advise you to do if you use a glass vessel, namely, warming it slowly and gradually; and in repeating this experiment, do as I do--pour the liquid out gently, leaving all the dirt behind in the basin: and remember that the more carefully and quietly you make this experiment at home, the better the crystals. To-morrow you will see the particles of alum drawn together; and if I put two pieces of coke in some part of the solution (the coke ought first to be washed very clean, and dried), you will find to-morrow that we shall have a beautiful crystallisation over the coke, making it exactly resemble a natural mineral. Now, how curiously our ideas expand by watching these conditions of the attraction of cohesion!--how many new phenomena it gives us beyond those of the attraction of gravitation! See how it gives us great strength. The things we deal with in building up the structures on the earth are of strength (we use iron, stone, and other things of great strength); and only think that all those structures you have about you--think of the “Great Eastern,” if you please, which is of such size and power as to be almost more than man can manage--are the result of this power of cohesion and attraction.