312 RESPIRATION. 



slices round the door, which serve for the entrance of air, and 

 by the tube, which leads to the meter. The quantity of air 

 which is drawn through it by this tube amounts to about 

 20,000 litres (TOG. 4 cubic feet) per hour, a quantity not merely 

 abundantly sufficient for ventilation, but to prevent loss or 

 error by diffusion into the air through the chinks round the 

 door. It is quite unnecessary to describe the aspirating appa- 

 ratus excepting in so far as to state that the clockwork is 

 moved by a weight, which, by a well-known mechanical con- 

 trivance, is constantly wound up by a steam-engine. 



To obtain a result, we must be able to determine with accu- 

 racy, first, the duration of the period of observation, and, 

 secondly, the quantity of carbonic acid gas contained in the 

 air which passes out of the chamber during that period. The 

 latter object may be attained either by estimating the total 

 weight of carbonic acid discharged, or by measuring the 

 volume of air aspired, and, simultaneously, the proportion by 

 volume of the same gas contained in it. In the apparatus 

 above described, the quantity of air discharged is so large 

 that it would not be possible to analyze the whole of it, so 

 that the second of the two alternatives must be adopted. 

 This is effected not by taking one or more specimens of the 

 discharged air from time to time and analyzing them (for this 

 plan, unless a very great number of analyses were made, would, 

 in consequence of the constant irregularities which occur in 

 the rate of discharge, give wrong results), but by causing a 

 definite proportion of the used air to pass through an absorb- 

 ing apparatus, and measuring the total quantity of carbonic 

 acid gas contained in it by a volumetrical method to be imme- 

 diately described. This division of the aspired air into two 

 parts, one to be measured and analyzed, the other merely to 

 be measured, is a matter of great difficult3 r ; for it obviously 

 involves the carrying on during the period of observation of 

 two continuous measurements i. e., the employment of two 

 meters instead of one, each of which must give results which 

 are not only accurate in themselves, but must correspond 

 exactly with those of the other. As, in applying the method 

 to animals so small that the whole quantity of air can be 

 analyzed, this difficulty is not met with, it is not necessary to 

 say anything as to the means of obviating it, or the errors 

 which, in spite of all precautions, it occasions. 



98. Application of Pettenkofer's Method to the 

 Determination of the Discharge of Carbonic Acid. 

 Gas in Small Animals. The apparatus consists of a metal 

 chamber of iron, which communicates in one direction with 

 the Bunsen's water air-pump ; in the other, with the apparatus 

 for the absorption of carbonic acid gas. Its lid closes air- 

 tight by means of a mercurial joint. For a guineapig or rat, 



