2i8 SKIN, HAIR, AND COLOUR, 



which have white pasterns, are more Hable to get cracked 

 heels than those which have dark ones. The supposed 

 idiosyncrasy in this case is no doubt chiefly due to the extra 

 amount of washing which the white pasterns receive, so as 

 to give them a clean appearance. 



Colour in relation to Heat and Cold. — Experience in 

 tropical climates teaches us that the darker a horse's coat is, 

 the better will he stand the effects of the sun. I may explain 

 that dark surfaces radiate out heat and also absorb it faster 

 than those of a lighter hue, as we may prove by the two 

 following experiments. If we fill with boiling water two 

 crockery teapots of the same form — one being white, the 

 other black — we shall find that if they are both placed in a 

 cool spot, the latter will lose its heat quicker than the 

 former. If, however, they are filled with icy cold water, and 

 are then exposed under similar conditions to the effect of the 

 sun on a hot day, the black one will get warm in a shorter 

 time than its fellow. We are also aware that the skin of 

 natives of tropical climates is darker than that of in- 

 habitants of temperate or cold countries ; and that in the 

 Arctic regions, the coats of various animals turn white on 

 the approach of winter. The working of this natural law is 

 most beneficent ; for, as the temperature of the body of the 

 polar bear, for instance, is much higher than that of the 

 atmosphere in which he lives, he can lose nothing on account 

 oi his whkt coat hdng a bad absorber of heat ; while the 

 fact of its parting with (radiating) heat slowly, helps the body 

 to maintain its normal degree of warmth. The skin of the 

 negro, for example, although it absorbs heat quickly, radiates 

 it still faster ; for the vapour given off by the skin cools the 

 surface, and also, by the mechanical protection it affords, 

 tends to prevent the absorption of heat from the atmosphere. 

 At first glance, we might, possibly, imagine that if the 

 surrounding air was warmer than the internal temperature 

 (about 99.5° Fahrenheit for the horse), no cooling effect 

 could be produced by radiation. When, however, the skin 



