COHESION 39 



mixed with it, the ice in the pot melted (they did not tell me 

 any thing about the salt and they set the pot by the fire, just 

 to make the result more mysterious), and in a short time the 

 pot and the stool were frozen together, as we shall very 

 shortly find it to be the case here, and all because salt has the 

 power of lessening the attraction between the particles of ice. 

 Here you see the tin dish is frozen to the board ; I can even 

 lift the little stool up by it. 



This experiment can not, I think, fail to impress upon your 

 minds the fact that whenever a solid body loses some of 

 that force of attraction by means of which it remains solid, 

 heat is absorbed; and if on the other hand we convert a 

 liquid into a solid, e. g., water into ice, a corresponding 

 amount of heat is given out. I have an experiment showing 

 this to be the case. Here (FiG. 21) is a bulb, A, filled with 



FIG. 



air, the tube from which dips into some colored liquid in 

 the vessel B. And I dare say you know that if I put my 

 hand on the bulb A, and warm it, the colored liquid which 

 is now standing in the tube at C will travel forward. Now 

 we have discovered a means, by great care and research into 

 the properties of various bodies, of preparing a solution of a 

 salt( 14 ) 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 



14 Solution of a salt. Acetate of soda. A solution saturated, or nearly 

 so, at the boiling point, is necessary, and it must be allowed to cool, ana 

 remain at rest until the experiment is 



