COHESION 27 



which I advise you to do if you use a glass vessel, namely, 

 warming it slowly and gradually; and in repeating this ex- 

 periment, 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 crystallization over the coke, mak- 

 ing 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 resjlt of this power 

 of cohesion and attraction. 



I have here a body in which I believe you will see a 

 change taking place in its condition of cohesion at the mo- 

 ment it is made. It is at first yellow; it then becomes a 

 fine crimson red. Just watch when I pour these two liquids 

 together both colorless as water. [The lecturer here mixed 

 together solutions of perchloride of mercury and iodide of 

 potassium, when a yellow precipitate of biniodide of mercury 

 fell down, which almost immediately became crimson red.] 

 Now there is a substance which is very beautiful, but see 

 how it is changing color. It was reddish-yellow at first, 

 but it has now become red (*). I have previously pre- 

 pared a little ol this red substance, which you see formed 



T Red precipitate of biniodide of mercury. A little care !s necessary to 

 obtain this precipitate. The solution of iodide of potassium should be 

 added to the solution of perchloride of mercury (corrosive sublimate) very 

 gradually. The red precipitate which first falls is redissolved when the 

 liquid is stirred: when a little more of the iodide of potassium is added a 

 pale red precipitate is formed, which, on the farther addition of the iodide, 

 changes into the brilliant scarlet biniodide of mercury. If too much iodide 

 of potassium is added, the scarlet precipitate disappears, and a colorless 

 solution is left. 



