EFFECT OF HEAT ON VOLUME 55 



to jump at once from a solid to a gas. Such a change of state is 

 called sublimation (89). At low temperatures, even ice may 

 sublimate. When gases of this kind are sufficiently cooled, 

 they return directly to the solid state. Under proper condi- 

 tions however, these substances may also be made to assume 

 the liquid state. 



53. Expansion of Gases. If sufficient heat is added to a 

 liquid it becomes a gas. When water assumes this condition, 

 we say it has evaporated. While liquids are known to occupy 

 more space than the solids from which they were made, the 

 most conspicuous examples of increase of volume with change 

 of state are found among the gases. Water turned to gas 

 (steam) increases more than 1600 times its volume. Engineers 

 roughly express it by saying that "A cubic inch of water makes 

 a cubic foot of steam." Another familiar example of a solid 

 that takes up much space when turned to gas is gunpowder. 



54. Effects of Withdrawing Heat. Withdrawing heat from 

 a substance has exactly the opposite effect upon its volume 

 that heating it has. All gases may be made liquid by with- 

 drawing heat, but the process is hastened by pressure. With a 

 reduction in temperature there always goes a reduction in 

 volume, and this reduction continues through the liquid and 

 solid states of practically all substances. Water, however, is 

 a conspicuous and important exception. Solid water (ice), like 

 other solids, contracts with a lowering of the temperature, and 

 liquid water contracts like other liquids, but just before the 

 point at which it turns from a liquid to a solid is reached 

 (about 4C.), it begins to expand, and as it turns to a solid, 

 it exerts a pressure of more than 100 tons to the square foot. 

 This pressure is sufficient to burst water pipes, split open 

 rocks, and disturb the foundations of buildings. It is to be 

 observed that the pressure is exerted only at the instant of 

 becoming solid. After this condition is reached, ice contracts 

 with loss of heat, as other substances do. 



