COHESION WEAKENED BY HEAT. 59 



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 learnt that 

 it makes calcareous spar, alum, and so forth, 

 crystallise 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 alto- 

 gether. 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 ! but what 

 does soft mean? Why, that the attraction 

 between the particles is so weakened that it is 

 no longer sufficient to resist the power we bring 

 to bear upon it. [Mr. Anderson handed to the 

 Lecturer the iron rod, with one end red-hot. 



