THE CHEMISTRY OF EXTERNAL RESPIRATION 239 



(1) Breathing into one spirometer and out of another, the in'spired 

 and expired air being directed by valves. The contents of the spiro- 

 meters are analyzed at the end of the experiment (Speck). In the 

 arrangement of Zuntz and Geppert, instead of the whole of the expired 

 air, a sample is collected for analysis during the entire duration of the 

 experiment, while the total volume expired is measured by a gas-meter. 

 This is a very convenient method for observations on man, especially 

 in disease, but each experiment can only be carried on at most for 

 fifteen to twenty minutes. 



(2) A small apparatus, much on the same principle, was used for 

 rabbits by Pfliiger and his pupils. A cannula in the trachea was con- 

 nected with a balanced and self-adjusting spirometer containing oxygen, 

 and the inspired and expired air separated by potassium hydroxide 

 valves, which absorbed 'the carbon dioxide. The amount of oxygen 

 used could be read off on the spirometer, and the amount of carbon 

 dioxide produced estimated in the liquid of the valves. 



(3) Elaborate arrangements, such as Pettenkofer's great respiration 

 apparatus, and the still larger and more efficient modifications of it 

 constructed since his time, in which a man, or even several men, can 

 remain for an indefinite period, working, eating, and sleeping. Air is 

 drawn out of the chamber by an engine, its volume being measured 

 by a gas-meter. But as it would be far too troublesome to analyze 

 the whole of the air, a sample stream of it is constantly drawn off, which 

 also passes through a gas-meter, through drying-tubes containing 

 sulphuric acid, and through tubes filled with baryta water. The baryta 

 solution is titrated to determine the quantity of carbon dioxide; the 

 increase in weight of the drying tubes gives the quantity of aqueous 

 vapour. A similar sample stream of the air before it passes into the 

 chamber is treated exactly in the same way, and from the data thus got 

 the quantity of carbon dioxide and aqueous vapour given off can readily 

 be ascertained. The oxygen can be calculated, as the difference be- 

 tween the final body-weight and the original body-weight plus the 

 weight of the carbon dioxide and water eliminated, but may also be 

 directly estimated by special methods. 



(4) Haldane and Pembrey have elaborated a gravimetric method, 

 which is very suitable tor small animals. It depends upon the absorp- 

 tion of carbon dioxide by soda lime. (See Practical Exercises, p. 299.) 

 In Atwater's so-called respiration calorimeter, which will be referred to 

 again under ' Animal Heat,' and by which, not only the gaseous metab- 

 olism, but the heat production can be measured in man, the carbon 

 dioxide is estimated in the same way. 



Inspired and Expired Air. The expired air is at or near the body 

 temperature, and is saturated with 'watery vapour. In ordinary 

 breathing it contains about 4 pej^ent. of carbon dioxide, while the 

 inspired air only contains a trace. The expire^air contains 16 or 

 17 per cent, of oxygen^ the inspired air about ^21 per cent. The 

 percentage of carbon dioxide in the alveolar air is, of course, greater 

 than in the ordinary expired air, since the relatively pure air of the 

 dead space constitutes a substantial fraction of the tidal air. The 

 carbon dioxide percentage in the alveolar air at the end of expira- 

 tion, with the body at rest, is remarkably constant in one and the 

 same individual at constant atmospheric pressure (p. 261). There 

 arc in addition in expired air small quantities of hydrogen and marsh- 



