in Energy, the Power of Nature 43 



of heat on matter by supposing radiant heat to be supplied 

 to a mass of I Ib. of ice at o F. Each unit of heat raises 

 the temperature of the mass by 2 (hence the capacity for 

 heat of ice is only half that of water), and by the time 1 6 

 units of heat have been absorbed, the mass of ice has ex- 

 panded considerably, and its particles are vibrating with 

 increased energy so that the temperature is 32. The next 

 144 units of heat which enter the mass produce no effect 

 on the temperature, which remains at 32. But the energy 

 is doing other work, for when the 144 units have been 

 absorbed we are dealing with water, not ice. Those 144 

 units have been expended in work against cohesion and are 

 stored up as potential energy. The heat employed in doing 

 this work of separating particles is sometimes said to 

 become latent, and the latent heat of water, i.e. the amount 

 of heat necessary to change I Ib. of the solid into i Ib. of 

 the liquid substance, is 144 F. heat-units. This is higher 

 than the latent heat of any other substance known. 



70. Action of Heat on Water. The volume of i Ib. 

 of water at 32 is 8 per cent less than the volume of i Ib. 

 of ice. This is a very significant fact, for almost all 

 other substances occupy a greater volume in the liquid than 

 in the solid state. When 7 heat-units are absorbed by i Ib. 

 of water at 32 the temperature rises to 39, but the volume 

 continues to diminish, a state of things which appears to 

 show that in water, unlike almost all other liquids, the faster 

 moving particles fit in a smaller space. But after 39 is past 

 each fresh unit of heat raises the temperature by about i, 

 and the volume of the liquid increases faster and faster. 

 From 32 the addition of 180 heat-units raises the tempera- 

 ture to 212 at ordinary atmospheric pressure; but here 

 another change takes place, and the water is said to boil. 

 No less than 967 units of heat must be supplied before the 

 temperature of i Ib. of water rises above 212, and at the 

 end of that operation there is not water but i Ib. of steam 

 or water- vapour at 212. A real experiment would not 

 proceed so regularly, because at all temperatures water, 

 and even ice, are partly converted into vapour, to produce 

 which a certain amount of heat is used up. 



