PHYSICAL FORCES 91 



stance which, for scientific purposes, may be conceived 

 of as being incompressible as we suppose solids to be 

 while its particles are incapable of sliding freely over 

 each other, there being a partial resistance thereto. 



With these changes of state, changes of volume do 

 not always correspond. As a rule, a solid increases in 

 volume in liquefying, and again increases in assuming 

 the aeriform condition, while it shrinks as the process is 

 reversed. With water, however, it is not so. In becom- 

 ing solid (ice), it increases in bulk, and becomes lighter 

 than in the liquid condition. 



But there are certain notable facts with regard to 

 heat in relation to such changes of condition. Heat 

 curiously disappears during changes from solidity to 

 liquidity, and from liquidity to gaseousness, but re- 

 appears in changes from, gaseousness to liquidity, and 

 from liquidity to solidity. 



Ice will absorb a larger amount of heat without indi- 

 cating any rise of temperature, until the whole of the 

 ice is melted. The heat so absorbed is commonly called 

 heat of liquefaction or latent heat, as distinguished from 

 heat which makes itself manifest. It is also called 

 potential heat, because it can reappear. It is thus again 

 evolved and " reappears " when a substance passes from 

 a liquid to a solid condition. 



If water be boiled, and so raised to a temperature 

 of 2 1 2, it will not show any higher temperature while 

 exposed to ordinary atmospheric pressure, even though 

 an amount of heat be applied to it enough to raise it 

 970. In this way much heat again disappears and 

 becomes potential, reappearing and becoming actual and 

 energetic once more, when vapour condenses into the 

 fluid condition. 



Mercury freezes at a temperature 38 below zero, or 



