490 



SCIENCE 



[N. 8. Vol. XXXIII. No. i 



the vapor form) is a gas and obeys all the 

 laws of gases. 



5. Air has only the same capacity for carry- 

 ing any vapor (water vapor) that any other 



.gas has. 



6. The amount of humidity {i. e., the basis 

 for calculating percentage humidity) that air, 

 or any other gas, can carry is not a function 

 of the nature of the gas at all. 



7. The amount of any vapor that any gas 

 can carry depends solely upon the nature of 

 the substance vaporized and the temperature. 



8. The actual percentage by volume of 

 vapor which any gas (air) can carry, i. e., the 

 saturation point, is determined by the follow- 

 ing formula: 



Per cent. = Pt./Atm. 



Where Pt. = vapor tension of the liquid at 

 temperature t, and Atm. = pressure of the at- 

 mosphere. The vapor pressure (the force 

 which a substance exerts to take the vapor 

 form) of water and most common liquids is 

 given in physical tables for all ordinary tem- 

 peratures. 



For example, at 20° C. (68° F.) water has 

 a vapor tension of 17.363 millimeters of Hg. 

 Air saturated with water vapor at 20° C, 

 therefore, contains 2.28 per cent. If it only 

 contains 1.14 per cent., its humidity is only 

 50 per cent. At 0° C. (32° F.) the vapor ten- 

 sion of water is 4.569 mm. and air saturated 

 at 0° C, therefore, carries only .6 per cent, 

 water vapor, or considerably less than if only 

 50 per cent, saturated at 20° C. 



9. Strictly, air is not altered by increase in 

 temperature so that it can carry more water 

 vapor, but the liquid producing the vapor is 

 changed so that it has a greater vapor tension 

 and a proportionately greater percentage of 

 vapor will enter the air. 



10. The rate of evaporation then from any 

 given moist surface depends upon the actual 

 amount of water the air is capable of taking 

 up (not on the percentage of unsaturation) 

 and upon the relative rapidity of movement of 

 the air. 



For example, from a square centimeter of 

 skin surface with saturated air no evaporation 



can take place at any temperature. At 20° C, 

 however, and 50 per cent, humidity, let us say 

 that 1.14 milligrams are vaporized per minute, 

 then at 0° C. and 50 per cent, humidity only 

 .3 milligram would be vaporized in the same 

 time. Even absolutely dry air at 0° C. evapo- 

 rates less moisture than at 20° C. and 70 per 

 cent, saturated. This is on the assumption 

 that the skin is equally moist at both tempera- 

 tures and that there is equal movement or no 

 movement of air in both cases. 



The amount of moisture that may be in the 

 air, whether greater or less, can have no 

 physiological effect, for that already there can 

 be considered a dry gas. There need only be 

 considered whether or not conditions are such 

 that much or little or none can be evaporated. 



11. Perhaps the normal human organism 

 requires the removal of the heat produced by 

 mental or physical activity at a certain fairly 

 definite rate. In such case both heat carried 

 away by radiation and by evaporation must be 

 considered, one in a measure supplementing 

 the other. Low temperatures increase radia- 

 tion and decrease heat loss by evaporation and 

 vice versa. 



For example, one extreme condition would 

 be when the air is saturated with water vapor 

 and at or above the body temperature. The 

 body then has lost all temperature control. 

 At any lower temperature there still remains 

 radiation though the air be saturated. 



The other extreme condition would be for 

 extremely low temperatures where, though 

 there is little cooling from evaporation and 

 little moisture produced for evaporation, 

 radiation is excessive and the only remedy is 

 the prevention of radiation by additional 

 clothing. 



Fortunately at low temperatures when 

 radiation is high, there is little loss from the 

 latent heat of evaporation because air at low 

 temperatures, even if perfectly dry, can carry 

 away little water vapor. In addition to this 

 balance of effect from outside influences, there 

 is the most important physiological iniluence 

 of perspiration increasing or decreasing the 

 moisture to be evaporated. 



Is it not possible, therefore, that the best 



