WATER 



61 



bubbles of vapor form within the liquid, rise through it, and burst 

 on the surface. The temperature of the water (if it is pure), and 

 of the steam, are now found to be identical, and this thermome- 

 ter reading is marked 100 C., the boiling-point. Water is thus 

 the standard substance used in the graduating of thermometers. 

 When so used for fixing the temperature of 100, the atmospheric 

 pressure must be normal, that is 760 mm., for the temperature of 

 boiling is lower when the pressure is lower. 



When water has reached the boiling-point, the temperature 

 ceases to rise, and the heat supplied is used in changing the water 

 into steam. The evaporation of 1 gram of water consumes 537 

 heat units or calories (heat of vaporization). The melting of 1 

 gram of ice consumes 79 calories (heat of fusion). The calorie is 

 the average amount of heat required to raise 1 gram of water one 

 degree in temperature between and 100. 



The amount of heat required to raise the temperature of & 

 given mass of water one degree is greater than that 

 required for an equal mass of any other common 

 material. Hence, the temperature of the sea changes 

 more slowly, and within a smaller range, than that of 

 the rocks which compose the land. For this reason 

 the climate of islands surrounded by much water is 

 less variable from season to season within the year 

 than is that of the continents. 



Steam. The tendency of water to evaporate 

 at various temperatures is best measured by the 

 gaseous pressure it exercises. A little water intro- 

 duced into a barometric vacuum (B, Fig. 33) will 

 depress the mercury, and the difference in height is 

 a measure of the vapor pressure of the water. The 

 temperature may be changed by putting hot water into the tube 

 surrounding the barometer, and thus the increase in vapor pres- 

 sure with rising temperature may be shown. At the column 





FIG. 33 



