SOURCES OF HEAT. 215 



perature, whether surrounded by bodies that are hotter or 

 colder than they are themselves. The cause of this is due 

 to the action of vital heat, or the heat generated or ex- 

 cited by the organs of a living structure. 



The following facts illustrate this principle : — The explorers of the Arctic 

 regions, during the polar winter, while breathing air that froze mercury, still 

 had in them the natural warmth of 98° Fahrenheit above zero; and the 

 inhabitants of India, where the same thermometer sometimes stands at 

 3; 5° in the shade, have their blood at no higher temperature. Again, 

 the temperature of birds is not that of the atmosphere, nor of fishes that 

 of the sea. 



487. The cause of animal heat is undoubt- 

 cause of vital cdly duc to cliemical action ; — the result of 

 respiration and nervous excitation, 



jjg . jj. Growing vegetables and plants also possess, in a degree, 



Bess this prop- the property of maintaining a constant temperature within 

 '■"'^^ their structure. The sap of trees remains unfrozen when the 



temperature of the surrounding atmosphere is many degrees below the freez- 

 ing point of water. 



This power of preserving a constant temperature in the animal structure is 

 limited. Intense cold suddenly coming upon a man who has not sufficient 

 protection, first causes a sensation of pain, and then brings on an almost irre- 

 sistible sleepiness, which if indulged in proves fatal. A great excess of heat 

 also can not long be sustained by the human system. 



Each species of animal and vegetable appears to have a temperatare natural 

 and peculiar to itself, and from this diversity different races are fitted for dif- 

 ferent portions of the earth's surface. Thus, the orange-tree and the bird of 

 Paradise are confined to warm latitudes; the pine-tree and the Arctic bear, 

 to those which are colder 



When animals and plants are removed from their peculiar and natural dis- 

 tricts to one entirely diSerent, they cease to exist, or change their character 

 in such a way as to adapt themselves to the climate. As illustrations of this, 

 we find that the wool of the northern sheep changes in the tropics to a spe- 

 cies of hair. The dog of the torrid zone is nearly destitute of hair. Bees 

 transported from the north to the region of perpetual summer, cease to lay up 

 stores of honey, and lose in a great measure their habits of industry. 



Man alone is capable of living in all cUmates, and of migrating fi-eely to all 

 portions of the earth. 



Of all animals, birds have the highest temperature ; mammalia, or those 

 which suckle their yountr. come next ; then ampliibious animals, fishes, and 

 certain insects. Shell-fish, worms, and tlie like, stand lowest in the scale of 

 temperature. The common mud-wasp, in its chrysalis state, remains unfrozen 

 during the most severe cold of a northern winter ; the fluids of the body in- 

 stantly congeal, however, in a freezing temperature, the moment the case, 

 or shell which incloses it, is crushed. 



