486 PROCEEDINGS OP THE AMERICAN ACADEMY. 



The cooling by respiration in moist air is therefore about one eighth 

 of that in dry air at the same temperature. But this is not all the 

 heat lost by evaporation, nor the greater part ; the loss by the skin 

 is nearly twice that by the lungs under the same conditions. Here 

 also the same law holds, the greater the relative moisture the less 

 evaporation and consequently the less cooling. 



According to Lavoisier and Seguin, 900 grams of fluid per day 

 are discharged by perspiration, and 500 grams from the lungs, 

 making 1400 grams of fluid lost in twenty-four hours. The evapo- 

 ration of this quantity of water will consume 750 units of heat, or 

 about one fifth of all the heat produced in the body in twenty-four 

 hours. 



The production of heat in the animal body, and its maintenance at 

 a normal standard, are two of the most important processes in the 

 living organism. The two chief means for regulating the temperature 

 of the body are the skin and the lungs. Of these the most direct and 

 simplest is that by the cutaneous perspiration. The relations of these 

 organs to the atmosphere, therefore, are of great importance in the 

 question now under consideration. 



But the rate of evaporation and consequent cooling depends in great 

 measure on the aqueous vapor already in the atmosphere. That this 

 relative amount has a material influence on our individual comfort 

 there is no doubt. It is certain that on those days when the propor- 

 tion of humidity is greatest, even the healthiest feel an oppression and 

 languor, and that on other days when the humidity is less there is an 

 exhilaration of spirits and an increase of muscular energy. 



It is worth while, then, to recall the laws governing this aqueous 

 vapor, for it pervades the atmosphere, is one of the main causes of its 

 movements, and the only fluctuating ingredient in its composition. 



The evaporating power of air raised to a higher temperature is 

 increased. A quantity of air absolutely humid at 59° F. holds an 

 amount of vapor ecjual to g'^ of its weight ; at 8G^, ^^^ ! ^^ 113°, ^^ ; 

 at 140°, jL ; so that while the temperature advances in an arithmetical 

 progression, the vapor-difFusing power of the atmosphere rises with the 

 accelerating rapidity of a geometrical series having a ratio of two ; 

 with the same ratio, evaporation increases, and consequently the cooling 

 process. 



It is upon this play of forces in the aqueous vapor and the air, and 

 the movements they bring about, that we must rely for the comfort of 

 our patients in the heats of summer. It is not a question of changing 

 the temperature of the air ; practically, we cannot alter that nor its 



