METHODS OF MEASURING RESPIRATORY EXCHANGE 29 



Benedict (fig. 6, A), have in later years superseded all other forms. 

 They are very effective and can easily be made airtight. In many 

 earlier closed-space respiration apparatus mercury was used in the 

 circulating pumps. Mercury should, however, be absolutely avoided 

 in any part of a closed-space respiration apparatus. The confined 

 quantity of air will rapidly become saturated with mercury vapour, and 

 will cause pronounced toxic symptoms as found by Krogh [1906] in 

 experiments on birds and incubated eggs, and by Carpenter and Bene- 

 dict [1909] in experiments on man. It has been attempted by Seegen 

 and Nowak [1879], and later by Krogh [1906], to purify the circulating 

 air by the application of a red heat at some point of the circuit. When 

 mercury is avoided this precaution would appear to be superfluous and 

 against mercury it is not in all cases effective. 



The CO 2 absorbing devices at present in use in large closed-space 

 respiration apparatus are not very satisfactory. Zuntz and Gerhartz 

 [1913] employ a strong potash solution which is circulated by a 

 special pump through an absorbing tower in which it presents a very 

 large surface to the air. This method has the very important advantage 

 of presenting no resistance to the passage of air and therefore of minim- 

 izing the danger of leakage ; but the strongly alkaline fluid must be 

 very disagreeable to handle, and the determination of the carbon dioxide 

 taken up becomes very complicated. Samples of the solution are 

 taken out and analysed for CO 2 at the beginning and end of each ex- 

 perimental period, but the volume of the potash solution is continually 

 changing from absorption both of CO 2 and water and this factor also 

 has to be taken into account. 



Atwater and Benedict dry the air current completely by blowing 

 it through a specially constructed absorber with sulphuric acid (fig. 7)- 

 The CO 2 is thereupon absorbed by moist soda lime and the air dried 

 again in a sulphuric acid absorber. The amount of CO 2 absorbed 

 is determined by weighing the soda lime can and the second water 

 vapour absorber. The resistance of the H 2 O absorbers is very con- 

 siderable, and the pressure of the air in this part of the apparatus 

 therefore so high that very elaborate precautions have to be taken 

 against leakage. 1 The absorbers are very heavy, so that it requires a 

 large and at the same time very sensitive balance to weigh them with 

 sufficient accuracy (cri gr.). It has been alleged by Morgulis [1913] 

 that they cannot be relied upon to absorb the last traces of moisture 



1 It ought to be possible, however, to construct efficient H 2 O absorbers with a large 

 surface over which the air could pass freely. 



