THE CHEMISTRY OF RESPIRATION. 399 



while fasting or digesting, and so on ; but when it comes to 

 be decided what organs are concerned in each case in pro- 

 ducing the greater or less exchange, and how much of the 

 whole is due to each of them, the question is one far more 

 difficult to settle and still very far from completely answered. 



The Changes Produced in Air by being once Breathed. 

 These are fourfold changes in its temperature, in its mois- 

 ture, in its chemical composition, and its volume. 



The air taken into the lungs is nearly always cooler than 

 that expired, which has a temperature of about 36 0. (97 

 F.). The temperature of a room is usually less than 21 C. 

 (70 F.). The warmer the inspired air the less, of course, the 

 heat which is lost to the Body in the breathing process; its 

 average amount is calculated as about equal to 50 calories in 

 twenty-four hours; a calorie being as much heat as will raise 

 the temperature of one kilogram (2.2 Ibs.) of water one degree 

 centigrade (1.8 F.). 



The inspired air always contains more or less water vapor, 

 but is rarely saturated; that is, rarely contains so much but it 

 can take up more without showing it as mist ; the warmer air is, 

 the more water vapor it requires to saturate it. The expired 

 air is nearly saturated for the temperature at which it leaves 

 the Body, as is readily shown by the water deposited when it 

 is slightly cooled, as when a mirror is breathed upon; or by 

 the clouds seen issuing from the nostrils on a frosty day, 

 these being due to the fact that the air, as soon as it is cooled, 

 cannot hold all the water vapor which it took up when 

 warmed in the Body. Air, therefore, when breathed once, 

 gains water vapor and carries it off from the lungs; the 

 actual amount being subject to variation with the tempera- 

 ture and saturation of the inspired air: the cooler and drier 

 that is, the more water will it gain when breathed. On an 

 average the amount thus carried off in twenty-four hours is 

 about 255 grams (9 ounces). To evaporate this water in the 

 lungs an amount of heat is required, which disappears for 

 this purpose in the Body, to reappear again outside it when 

 the water vapor condenses. The amount of heat taken off in 

 this way during the day is about 148 calories. The total daily 

 loss of heat from the Body through the lungs is therefore 

 198 calories, 50 in warming the inspired air and 148 in the 

 evaporation of water. 



The most important changes brought about in the 



