CH. XXIV.] 



CHEMISTRY OF RESPIRATION. 



375 



of hydrogen, which, during metabolism, are oxidised to form 

 water ; a small amount of oxygen is also used in the formation 

 of urea. Carbohydrates contain sufficient oxygen in their own 

 molecules to oxidise their hydrogen ; hence the apparent loss of 

 oxygen is least when a vegetable diet (that is, one consisting 

 largely of starch and other carbohydrates) is taken, and greatest 



when much fat and proteid are eaten. The quotient &_2 



2 absorbed 



is called the respiratory quotient. Normally it is 3_? = 0-9, but 



it varies considerably with diet as just stated. It varies also with 

 muscular exercise as the output of carbonic acid is then increased 

 both absolutely and relatively to the amount of oxygen used up. 

 The amount of respiratory interchange of gases is estimated by 

 enclosing an animal in an air-tight chamber, except that there is 

 a tube entering and another leaving it ; by one tube oxygen or 



Fig. 332. Haldane's apparatus for estimating the carbonic acid and aqueous vapour given 



nil by an animal. 



air can enter and is measured by a gas-meter as it passes in. 

 The air is drawn through the chamber, and leaves it by the other 

 tube ; this air has been altered by the respiration of the animal, 

 and in it the carbonic acid and water are estimated ; the carbonic 

 acid is estimated by drawing the air through tubes containing a 

 known amount of an alkali ; this combines with the carbonic acid 

 and is increased in weight : the increase in weight gives the 

 amount of carbonic acid ; the alkali used in Regnault and Reiset's 

 apparatus was potash ; Pettenkofer used baryta water ; Haldane 

 recommends soda-lime. The water is estimated in tubes containing 

 pumice moistened with sulphuric acid. 



The accompanying drawing (fig. 332) shows the essential part 

 of the simple but effective apparatus used by Haldane. The 

 animal is placed in the vessel A ; air is sucked through the 

 apparatus (which must be perfectly air-tight) by a water pump 

 at a suitable rate. The arrows indicate the direction in which 

 the air passes. It goes first through two Woulff's bottles, i 

 and 2. No. i contains soda-lime, which frees the air from 



