ACTION OF THE TROPICAL SUN. VZl 



even the anatomic build of the latter, is not different in the tropical 

 races from that of white men. Daubler states that the negro has larger 

 and better developed sweat glands than the white man. The fact that 

 Eubner could not find greater water evaporation in negroes than in 

 Europeans at high air temperatures does not contradict my supposition, 

 for, if I am right, the dark skin is superior to the white only in the sun, 

 where radiated heat is absorbed more intensively by the one than the 

 other. 



Certain parts of the body, in brown as well as in white men, seem 

 to sweat earlier and more intensively than others. For instance, the 

 forehead always secretes sweat earlier than the arm. As a result, the 

 temperature on the forehead has begun to fall while that of the arms is 

 still rising. In the sun the ultimate temperatures observed on the 

 arm are generally higher than those on the forehead. 



The results obtained so far indicate that the temperature of the 

 human skin increases in the sun, but does not reach the normal body 

 temperature. In animals without sweat glands, the skin temperature 

 rises above febrile heights and the tissues lying underneath are heated. 

 On the other hand, penetration of heat through the human skin seems 

 improbable, the effect of the rays absorbed being neutralized by water 

 evaporation on the skin. The more perfect this water evaporation is, 

 the better the normal body temperature may be maintained. The be- 

 havior of normal dogs as compared with those which have undergone 

 tracheotomy shows this fact plainly. Monkeys exposed to the sun in 

 Manila die in little over one hour because of their limited capacity to 

 evaporate water, while man, with his well-developed sweat glands resists 

 the same climatic conditions for a much longer period without detriment. 



My experiments demonstrate the enormous physiologic and hygienic 

 importance of ample water evaporation in the Tropics. We are the 

 better off, the better we can lose heat by water evaporation. 



Water evaporation from the skin is the most complete when a large 

 part of the skin area is uncovered. The native laborer in the Tropics 

 generally wears but little clothing, often only a breeehclout. On the 

 other hand, the white skin can not withstand the direct rays of the sun. 

 Sunburn, erythema solare, or even more severe lesions, are produced by 

 the sun's rays, while such injuries rarely occur with the brown skin. The 

 range of the rays which produces this effect is not entirely known, but it 

 is to be presumed that they lie in the violet end of the spectrum and 

 beyond. Now, because of this effect the colored man can expose his 

 body to the tropical sun, but the white man must keep covered. Neces- 

 sarily, under the same climatic conditions, water evaporation from the 

 skin when uncovered is much more free than that from the same skin 

 covered with clothes. If we further consider that the colored skin 



