HEAT EVOLVED DURING SOLIDIFICATION. 65 



standing in the tube at c will travel for- 

 ward. Now we have discovered a means, by 

 great care and research into the properties of 

 various bodies, of preparing a solution of a salt( 15 ) 

 which if shaken or disturbed will at once become 

 a solid; and as I explained to you just now (for 

 what is true of water is true of every other 

 liquid), by reason of its becoming solid, heat is 

 evolved, and I can make this evident to you 

 by pouring it over this bulb; there! it is 

 becoming solid, and look at the coloured liquid, 

 how it is being driven down the tube, and how 

 it is bubbling out through the water at the 

 end; and so we learn this beautiful law of our 

 philosophy, that whenever we diminish the 

 attraction of cohesion we absorb heat and 

 whenever we increase that attraction heat is 

 evolved. This, then, is a great step in advance, 

 for you have learned a great deal in addition 

 to the mere circumstance that particles attract 

 each other. But you must not now suppose 

 that because they are liquid they have lost their 

 attraction of cohesion; for here is the fluid 

 mercury, and if I pour it from one vessel into 

 another, I find that it will form a stream from 

 the bottle down to the glass a continuous rod 



F 



