42 



is still boiling, and set the flask on one side to cool : it 

 is now found that the water deposits no crystals on cooling, 

 but remains perfectly clear; and, indeed, if the flask be 

 not shaken or exposed to great cold, the liquid will remain 

 quite free from crystals. 



In this state the water is said to be Supersaturated, for if we 

 remove the plug of wool from a solution, previously prepared 

 in the manner we have indicated, and a crystal of Alum be 

 dropped on the surface of the liquid immediately, crystals of 

 Alum grow rapidly on this nucleus, and to so great an extent 

 does this take place, that, in a minute, the liquid of the flask 

 has become apparently a solid mass of Alum crystals. 



This sudden crystallisation is attended by a large 

 evolution of heat, so great as to be appreciable by the touch ; 

 or if, on the introduction of the crystal of alum, the bulb of 

 a mercurial thermometer be placed in the liquid so soon as 

 crystallisation sets in, the mercurial column rises, indicating 

 an evolution of heat. 



At this stage, we may say, occurs a sudden manifes- 

 tation of heat, and so true and just is Nature, that we look 

 back into the history of solution to find whence this heat 

 comes. We have a large thermometer of special construc- 

 tion, consisting of a large, thin glass bulb, blown at the end 

 of a glass tube, of which the free end dips into a cistern of 

 coloured water. The bulb and tube is filled with air, and if, 

 by any means, we cool the bulb, the air contained in it 

 contracts, and the coloured liquid rises from the cistern into 

 the long tube, a contrary effect being observed if a hot body 

 is placed in contact with the bulb. Such an arrangement is 

 termed an Air Thermometer, and is far more sensitive to 

 heat and cold than the ordinary mercurial instrument, but 

 its bulk prevents its general use. Surrounding only the 

 bulb with a glass vessel, in which water can be placed, we 

 are now in a position to observe what heat changes take 

 place on dissolving a solid in water. 



If we pour cold water into the vessel, and wait until the 

 level of the coloured liquid, in the long tube, remains 



