6 5 8 THE POPULAR SCIENCE MONTHLY. 



instinctively try to increase the movement of the air that means, its 

 quantity in proportion to your surface ; you want to increase your 

 loss by conduction, and, if possible, by evaporation, and take to fan- 

 ning, in order to facilitate the dej^arture of your rising heat by the 

 two open routes. 



The loss by radiation can be very considerable under certain cir- 

 cumstances ; 50 per cent, of the whole quantity of heat generally go- 

 ing that way, it is obvious that radiation deserves our full consider- 

 ation. Particularly, an unequal radiation may be very injurious, such 

 as takes place when a person is sitting or lying near a cold wall which 

 is not covered by some bad heat-conductor, or near a window, etc. 



On school-forms, the exposed sides of the first and last pupils are 

 always more cooled than the sides directed toward their neighbors. 

 In this respect there are numbers of practical points which are far 

 from being sufficiently taken into consideration. 



Let us now consider some instances in which the abstraction of 

 heat by evaporation is predominant, or preeminently felt. The best 

 known is that experiment by which one tries to learn the direction 

 of the wind when the air appears calm and the sky cloudless. The 

 moistened forefinger feels colder on that side which looks toward the 

 wind, because more evaporation takes place there. The experiment 

 does not succeed so well when the air is moist, because the moisture 

 in the air prevents further reception of moisture by it ; in our case, 

 preventing the evaporation from the moistened finger. 



Our organism acts similarly in all cases where there is an increased 

 production of heat in our body, or where less heat is sent away by the 

 two other routes. It has the power of dilating or narrowing the small 

 blood-vessels in our skin and internal organs. The blood-vessel nerves 

 which govern this motion are not subject to our will, but liable to be 

 excited by external causes. When a person blushes, he gives off heat, 

 because more blood rushes into the dilated blood-vessels of his cheeks 

 and periphery generally, and more heat leaves the body. Under 

 similar circumstances the whole surface of our body becomes fuller 

 of blood and warmer, there is more heat to radiate and to be con- 

 ducted away, and to be consumed by increased evaporation of the 

 watery part of the blood. 



The great value of evaporation for the cooling of our body can be 

 estimated by calculating that as little as fifteen drops of water re- 

 quires 2^ caloric units to be changed into vapor. 



We have at Munich a great apparatus for studying the process of 

 respiration. It was given by the late King of Bavaria, Maximilian 

 II., to the hygienic department of the university. Prof. Voit and my- 

 self have, by aid of this apparatus, investigated the quantity of water 

 evaporated by men and animals during twenty-four hours. The con- 

 stant result was that, under other similar circumstances, the quantity 

 of evaporated water always rose in proportion to an increased meta- 



