32 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



the quantity of enclosed air to give the quantity of each separate gas. 

 The quantity of enclosed air is the known volume of the apparatus re- 

 duced to standard conditions (o, 760 mm. dry pressure). Zuntz and 

 Oppenheimer [1908] carry out the reduction automatically by means 

 of a so-called thermo-barometer, in which loo c.c. of dry air at o and 

 760 mm. have once for all been enclosed. The apparatus consists of 

 a long tube arranged in the animal chamber. The inside of the tube 

 is moistened, and the volume of the enclosed air at the pressure ob- 

 taining can be read off from outside. 100 divided by the observed 

 volume gives at once the reduction factor. As pointed out by Krogh 

 [1908] this apparatus is very unreliable. The chances are against its 

 representing the actual average temperature of the chamber, and the 

 moisture will in the course of time condense at the point where the 

 temperature is lowest and leave the warmer parts more or less dry. 

 There can be ; no doubt that the best plan is to mix the air in the 

 animal chamber thoroughly and continuously by means of an electric 

 fan, as done by Grafe and by Benedict and Carpenter, and to measure 

 the temperature on thermometers in one or more places. The degree 

 of humidity can be measured on a pair of wet and dry bulb thermo- 

 meters placed in the tube, by which the ventilating air current leaves 

 the chamber, as done by Stahelin and Kessner and by Zuntz [1913] 

 in his new 80 cub. metre Regnault apparatus. The pressure should 

 always be kept absolutely equal to the barometric. 



i. A. (V) Air-Current Respiration Apparatus. 



As mentioned above, the air-current respiration apparatus fall 

 naturally into two groups : 



a. Those in which the ventilating air current is not measured but 

 treated as a whole with absorbing reagents. These are obviously suit- 

 able for comparatively small animals only. 



ft. Those in which the ventilating air current is accurately measured, 

 while only a representative sample is drawn off and subjected to analysis. 

 These are suitable for animals of the largest size. 



a. The simplest form of an air-current apparatus is that which is 

 suitable for measuring the CO 2 output of very small animals and 

 which is extensively used in experiments with plants (fig. 8). The 

 air current which can be provided by an aspirator is deprived of car- 

 bon dioxide by means of soda lime and tested with baryta solution. 

 After passing through the animal chamber it is taken through baryta 

 bottles or Pettenkofer's baryta tubes in which the carbon dioxide pro- 



