RESPIRATION. 143 



air is cold, it is pure ; but this is an error, since cold air 

 will receive and retain the same impurities as warm air. 



44. Shall we open our bedrooms to the night air ? Flor- 

 ence Nightingale says, in effect, that night air is the only 

 air that we can then breathe. "The choice is between pure 

 air without and impure air within. Most people prefer the 

 latter, an unaccountable choice. An open window, most 

 nights in the year, can hurt no one. In great cities, night 

 air is the best and purest to be had in twenty-four hours. 

 I could better understand, in towns, shutting the windows 

 during the day than during the night." 



45. Animal Heat. Intimately connected with respi- 

 ration is the production of animal heat, or the power of 

 maintaining the temperature of the body above that of the 

 medium in which the creature moves; thus, the bird is 

 warmer than the air, and the fish than the water. This 

 elevation of temperature is a result of the various chemi- 

 cal changes which are constantly taking place in the system. 

 Although common to all animals, in a greater or less 

 degree, heat is not peculiar to them ; since plants also gen- 

 erate it, especially at the time of sprouting and flowering. 

 If a thermometer be placed in a cluster of geranium flow- 

 ers, it will indicate a temperature several degrees above 

 that of the surrounding air. 



46. Among animals great differences are noticed in this 

 respect, but the degree of heat produced is always propor- 

 tional to the activity of respiration and the amount of 

 oxygen consumed. Accordingly, the birds, whose habits 

 are extremely active, and whose breathing capacity is the 

 greatest, have uniformly the highest temperature. Slug- 

 gish animals, on the contrary, as frogs, lizards, and snakes, 

 have little need for oxygen, and have incompletely de- 



44. State what Florence Nightingale says about inhaling night air? 



45. Warmth of the bird as compared with that of the air? Of the fish and 

 the water ? Heat in animals and plants ? How illustrated with the thermometer ? 



46. Amount of heat in animals, how apportioned? As regards the birds? 

 Frogs, and other sluggish animals ? Arrangement made by zooligsts ? 



