LECTURE III 



COHESIONCHEMICAL AFFINITY 



WE will first return for a few minutes to one of the 

 experiments made yesterday. You remember 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 number of beautiful crystals. Here also are the pieces of 

 coke which I 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 crystallize 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 attraction which the particles of alum have 

 for each other. 



And now for another experiment. We have already 

 gained a knowledge of the manner in which the particles 

 of bodies of solid bodies attract each other, and we have 

 learned that it makes calcareous spar, and so forth, crystal- 

 lize in these regular forms. Now let me gradually lead 

 your minds to a knowledge of the means we possess of 

 making this attraction alter a little in its force; either of 

 increasing, or diminishing, or, apparently, of destroying it 

 altogether. I will take this piece of iron [a rod of iron about 

 two feet long and a quarter of an inch in diameter]. It 

 has at present a great deal of strength, due to its attraction 

 of cohesion; but if Mr. Anderson will make part of this 

 red-hot in the fire, we shall then find that it will become soft, 

 just as sealing-wax will when heated, and we shall also 

 find that the more it is heated the softer it becomes. Ah I 



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