258 WATER ITS RELATION TO METABOLISM 



at 85 F. 1278. If air saturated at 85 F. be cooled to 60 F. every 

 c. ft. of air will yield 7 grains of water. Vaporisation is produced 

 by the shooting forth of water molecules which are separated from 

 the free surface by their vibratory action. The rapidity and force 

 of these molecular nights is greater in proportion to the heat of the 

 molecules. The full saturation of a vacuous space is brought about 

 far more quickly than one containing air, for the air particles, by 

 crossing the path of the vapour molecules, retard the penetration of 

 the aqueous molecules among them. When air drifts in conditions 

 of wind it carries the floating vapour with it and accelerates eva- 

 poration. The rate of evaporation of the sweat then depends upon 

 the relative humidity of the atmosphere, and upon the temperature, 

 movement, and pressure of the air. 



The relative humidity is obtained by dividing the weight of 

 vapour actually existing in the atmosphere by the weight of vapour 

 which would be present if the air were saturated. The dew-point 

 and the actual temperature must first be observed, then the tension 

 of aqueous vapour (a) at the temperature of the dew-point, and (b) at 

 the actual temperature of the atmosphere can be found in tables. 



Then relative humidity = - x 100. The dew-point can be directly 



found by running iced water through a chamber covered with 

 black glass. The chamber contains a thermometer, and the tem- 

 perature is read when dew forms on the glass. The atmosphere is, 

 on the average, about 75 per cent, saturated in this country. The 

 relative humidity is greatest near the surface of the earth at night 

 when the temperature approaches the dew-point. It is also great 

 in the morning when the sun has evaporated the dew, and the 

 vapour has not diffused upwards. 



In winter the saturation may be 75 per cent, at 2 P.M. and 95 

 per cent, at midnight, while in summer the saturation at 2 P.M. may be 

 50 per cent, and at 6 A.M. 75 per cent. In any one day the variations 

 may be much greater. At the seaside the daily variations in satura- 

 tion are much smaller. Air in forests is 10-20 per cent, moister 

 than air in the open. In California the saturation drops from 100 

 per cent, at dawn to 22 per cent, at noon. The Fohnwind when 

 it reaches the Riviera lowers the saturation 50 to 60 per cent, in 

 an hour or two. The difference in saturation between out of doors 

 and a stove-heated room in winter may be 50 to 80 per cent. 



Water vapour is a far better conductor of heat than dry air, and 

 thus air saturated with vapour at 35 F. is raw and chill, and damp 

 houses and clothing increase the loss of body heat. The beauty of 

 earth and sky, the glories of sunrise and sunset, depend upon the 

 particles of dust and frozen vapour in the atmosphere. The particles 



