COHESION 37 



but what does soft mean? Why, that the attraction be- 

 tween the particles is so weakened that it is no longer suf- 

 ficient to resist the power we bring to bear upon it. [Mr. 

 Anderson handed to the lecturer the iron, rod, with one end 

 red-hot, which he showed could be easily twisted about with 

 a pair of pliers.] You see I now find no difficulty in bending 

 this end about as I like, whereas I can not bend the cold 

 part at all. And you know how the smith takes a piece 

 of iron and heats it in order to render it soft for his 

 purpose: he acts upon our principle of lessening the adhesion 

 of the particles, although he is not exactly acquainted with 

 the terms by which we express it. 



And now we have another point to examine, and this 

 water is again a very good substance to take as an illustra- 

 tion (as philosophers we call it all water, even though it be 

 in the form of ice or steam). Why is this water hard? 

 [pointing to a block of ice]*; because the attraction of the 

 particles to each other is sufficient to make them retain 

 their places in opposition to force applied to it. But what 

 happens when we make the ice warm? Why, in that case 

 we diminish to such a large extent the power of attraction 

 that the solid substance is destroyed altogether. Let me 

 illustrate this : I will take a red-hot ball of iron [Mr. Ander- 

 son, by means of a pair of tongs, handed to the lecturer a 

 red-hot ball of iron, about two inches in diameter], because 

 it will serve as a convenient source of heat [placing the red- 

 hot iron in the centre of the block of ice]. You see I am 

 now melting the ice where the iron touches it. You see the 

 iron sinking into it; and while part of the solid water is 

 becoming liquid, the heat of the ball is rapidly going off. 

 A certain part of the water is actually rising in steam, the 

 attraction of some of the particles is so much diminished 

 that they can not even hold together in the liquid form, 

 but escape as vapor. At the same time, you see I can not 

 melt all this ice by the heat contained in this ball. In the 

 course of a very short time I shall fend it will have become 

 quite cold. 



Here is the water which we have produced by destroying 

 some of the attraction which existed between the particles 

 of the ice, for below a certain temperature the particles of 



