ACTION OF THE TROPICAL SUN. 103 



on 1,000 American soldiers in the Philippines. The results, as to the 

 advantage of orange-red, were negative. This fact, when considered in 

 connection with a niunber of observations which I have made during 

 my stay in the Tropics, convinced me that the rays of the tropical sun 

 having greater wave length, that is, those in the red and ultra-red 

 end of the spectrum, play the most important role in producing the 

 untoward effects generally attributed to tropical sunlight. 



In making this statement it must be understood that it refers to 

 organisms having the capability of regulating the body heat, and not 

 to those low in the scale, such as bacteria or protozoa, for it has 

 been shown repeatedly * that in the case of the latter ultra-violet 

 rays exert a most destructive action, heat coming into consideration 

 only in so far as such organisms are not able to live when the tem- 

 perature is above a certain point. Plants also, the normal life action 

 of which depends on the chlorophyll, of course are markedly affected 

 by the ultra-violet as well as by the other end of the spectrum. 



In order correctly to interpret the experiments given in subsequent 

 portions of this paper, it will be necessary briefly to review the physi- 

 ologic processes concerned in heat regulation in the bodies of mammalia. 



The body possesses the capability not only of regulating its heat pro- 

 duction from the combustion of foodstuffs (chemical heat regulation), 

 but also its loss of heat from convection, radiation and water evapora- 

 tion (physical heat regulation). Normally, the thermal effects of the 

 suri'oundings are compensated either by a suitable transference of heat 

 to the surroundings, or by the conservation or production of heat within 

 the body, so that the body temperature within narrow limits remains 

 practically the same. However, there are limits to the power of regula- 

 tion. If the body is heated too intensively or the loss of heat is inhibited, 

 the latter will accumulate and the body temperature rise. 



The higher the temperature of the surroundings, the less will be the 

 loss of heat by conduction or radiation, and if this temperature exceeds 

 that of the body, no heat can be lost in this way, but on the contrary 

 the balance is changed, and the energy lost to the body would now be 

 accumulated in it were it not for the loss occasioned by the evaporation 

 of water from the lungs and the surface of the body. 



High air temperatures alone do not change the body temperature as 

 long as the latter can be regulated by the loss of sufficient heat through 

 water evaporation.^ Therefore, a man can withstand temperatu.res even 

 of 189° for a considerable time if the air is comparatively free from 

 water vapor. On the other hand, if the relative humidity is high and, 



' hoc. cit. 



' Hill, Leonard, Recent Advances in Physiology and Biochemistry, London, 

 (1908), 256-274. 



