NATURAL WATERS 73 



Water is remarkable with respect to the influence of changes 

 of temperature upon its volume. Like most substances, it 

 expands when heated and contracts when cooled ; but a care- 

 ful examination will reveal the fact that this statement, though 

 roughly correct, is too general, and that at a certain tempera- 

 ture water is most dense and expands whether it be heated or 

 cooled. This temperature of maximum density is about 4° C. 

 When water changes into ice an expansion occurs (see chap. iii.). 

 In this respect water is unlike most other substances. It is 

 owing to water being most dense at 4° C. that the tempera- 

 ture of the water in deep lakes or ponds even in the coldest 

 weather is usually found to be about 4° at considerable depths 

 below the ice. 



Latent Heat. — When a liquid changes into a solid heat is 

 evolved, and, conversely, when a solid becomes liquid heat 

 is absorbed. 



If a quantity of ice or snow, say at - 10° C, be placed in 

 a vessel and heat be applied, a thermometer placed in the 

 material will show a slow rise in temperature until 0° C. is 

 reached, when, although the supply of heat be maintained, 

 no further rise will ensue until all the ice or snow is melted. 



The amount of heat thus absorbed is very considerable, and 

 is exactly equal to that which was evolved when the ice was 

 formed from liquid water. 



If akilogramme of waterat 80°C. be mixed with a kilogramme 

 of ice at 0° it will be found that the ice will be melted but the 

 temperature of the resulting liquid will only be 0°. It is thus 

 evident that to melt a kilogramme of ice without producing any 

 change of temperature as much heat is required as would raise 

 a kilogramme of water through 80° C, or 80 kilogrammes of 

 water through 1° C. Heat so absorbed is called latent, since it is 

 hidden, so far as a thermometer is concerned. Conversely, when 

 water freezes, each kilogramme converted into ice gives out 

 as much heat as would raise 80 kilogrammes of water through 

 1°0. It is for this reason that during cold weather ice onlv 



