30 CHEMISTRY FOR AGRICULTURAL STUDENTS 



due to this that evaporation of water takes place much more rapidly in hot 

 weather than in cold, and that warm air can absorb much more water 

 vapour than cold air before it becomes saturated. The amount of water 

 vapour that can be retained by air, therefore, depends upon the tempera- 

 ture. Water vapour is always present, and however apparently dry the 

 air maybe, a temperature is reached on cooling — the "dew-point" — at 

 which water vapour begins to condense in the form of mist or dew. This 

 temperature is most easily determined by the dew-point hygrometer, but it 

 can also be ascertained by calculation from the difference between a wet and 

 a dry bulb thermometer. These thermometers will record the same tem- 

 perature when the air is saturated, but the drier the air is the more will the 

 reading of the wet bulb fall below that of the dry, because greater evapora- 

 tion from the wet bulb takes place, and more heat is absorbed in conse- 

 quence (see p. 34). The deposition of dew when air is cooled is con- 

 veniently illustrated by bringing a flask of ice-cold water into a warm 

 room. 



The distillation of water that occurs in nature will now be understood. 

 Air into which water has evaporated from sea and land deposits its water 

 when cooled in the form of cloud, mist, or dew. From cloud, water is 

 precipitated as rain ; so that rain is distilled water, free from all dissolved 

 solids, and only containing the dissolved oxygen, nitrogen, and carbonic 

 acid gas of the atmosphere. These gases may be obtained for examination 

 by completely filling an apparatus, such as that figured, with rain water, 



Fig. 15. 



and heating the flask until the gases are expelled. When rain water sinks 

 through the soil and the underlying strata, it dissolves the soluble sub- 

 Stances ; so that river and spring water contain dissolved solids varying in 



