4 THE POPULAR SCIENCE MONTHLY. 



This is conveyed to the air by minute ducts passing through the epi- 

 dermis. It is obvious that, the blacker the pigment, the less light 

 and heat will be transmitted to excite these glands into activity. In 

 the second place, there is a continual transudation of sweat from the 

 minute vessels of the surface of the body through the epidermis at 

 every point. The thicker or the more oily the scarf-skin, the less will 

 the amount of this transudation be. If it be both thick and oily, as 

 in many dark races, the quantity transuded will be reduced to a mini- 

 mum ; if it be thin and not oily, as in the fairest members of the 

 white race, transudation will be copious. 



The amount of transuded sweat depends, however, not merely on 

 the thinness of the cuticle, but also on the degree to which the air in 

 contact with the body is saturated with moisture ; for there is a limit 

 to the quantity of vapor which the air can absorb. This limit varies 

 with the temperature, warm air absorbing more than cold. Such being 

 the nature of the skin, I now proceed to inquire what kind of it will 

 best suit particular regions. For this purpose climates may be classi- 

 fied as 



I. Arctic. 

 II. Temperate humid. 



III. Temperate dry. 



IV. Tropical humid. 

 V. Tropical dry. 



I. When the skin is exposed to great cold, perspiration by tran- 

 sudation is accelerated. The frosty air, raised many degrees in tem- 

 perature by contact with the body, becomes very dry, and greedily 

 drinks in its moisture. At the same time the body loses, not only the 

 heat which the air carries off, but also that which is rendered latent 

 by the evaporation of the sweat. As a protection against the injury 

 which a too rapid loss of perspiration and heat may inflict in an arctic 

 climate, a thick integument is desirable. On account of the obliquity 

 of the sun's rays a dark pigment will be a disadvantage, because it will 

 prevent the passage of light and heat. Some pigment will, however, 

 be needed, as not even in northern regions can albinoes expose them- 

 selves to sunlight with comfort. The coloring matter, then, will be 

 light ; but, owing to the thickness of the cuticle, the general effect 

 will be yellow. 



II. By a humid temperate climate I mean one occurring in a tem- 

 perate zone, in which the air constantly contains a large amount of 

 moisture. Humidity does not to any considerable extent depend on 

 the amount of the annual rainfall. The annual rainfall of London is 

 twenty and one half inches, that of Toronto thirty inches ; yet the air 

 of the former place is incomparably more humid. Countries in which 

 this climate is found are distinguished from others in the same latitude 

 by the limited range of the thermometer. This is due partly to the 

 fact that water can not be so rapidly heated as air, and partly to the 



