38 RESPIRATORY EXCHANGE OF AlS f IMALS AND MAN 



EXAMPLE. 



When the chamber is small compared with the volume of air pass- 

 ing through it during an experimental period, that is in experiments 

 of long duration (twenty-four hours), the possible changes in composition 

 of the air in the chamber will have a slight and usually negligible effect 

 only, but in short experiments it is necessary to supplement the average 

 samples with others taken direct from the chamber at the beginning and 

 end of each experimental period, at the same time measuring the tem- 

 perature, pressure and degree of humidity in it. The formulae neces- 

 sary to calculate the respiratory exchange in such cases have been 

 given by A. and M. Krogh [1913]. 



There is no doubt in the writer's opinion that the Jaquet apparatus 

 is the respiration apparatus of the future for normal experiments on 

 man and large animals, but at present it has this drawback that it is 

 difficult to determine oxygen with sufficient accuracy by gas analysis. 

 The limit of accuracy now attainable is about + 0*01 percent., and even 

 this is in most instruments a relative and not an absolute accuracy. 

 A short series of analyses of the same air may agree within these limits, 

 but unless the burette is always kept scrupulously clean, which takes 

 much time, analyses of the same air made at longer intervals are apt to 

 differ several hundredths. Though the composition of the atmospheric 

 air even in towns is constant as shown by Benedict [1912] to within 

 O'Oi per cent., it is therefore generally necessary in work with Jaquet's 

 apparatus to analyse both the ingoing and the outgoing air and to let 

 these analyses alternate regularly (A. and M. Krogh). 



i. B. Apparatus for Measuring the Pulmonary Gas Exchange. 



Special researches (Schierbeck [1893], Krogh [1904], Franchini and 

 Preti [1908]) have shown that the gas exchange taking place through 

 the skin in higher animals is very insignificant, as it is generally I per 

 cent, or less of the total gas exchange. Quite apart therefore from the 

 use of pulmonary respiration apparatus to study the processes taking 



