106 COMPARISONS OF RESPIRATORY EXCHANGE. 



dioxide will be larger than that actually produced. By means of a 

 large number of experiments, a system of controls has been introduced 

 which makes it practically impossible for such inefficiency to occur. 

 In these experiments the amount of water which it is possible for these 

 absorbers to retain has been carefully determined and it has been made 

 a part of the experimental routine to change the absorbers before they 

 reach this point. So long as this routine is followed, there will be no 

 danger of an incomplete absorption of the water-vapor. It must be 

 remembered that the rate of ventilation must be the same as that 

 when the efficiency tests were carried out. It is obvious that in all 

 experiments the air entering the soda-lime container must be dried to 

 the same degree of humidity as the air leaving its accompanying water- 

 absorber. Usually this means absolute desiccation, but with extremely 

 high rates of ventilation and with large quantities of water in the air, 

 slight traces of water-vapor may escape from the air-drier preceding 

 the carbon-dioxide absorber and an equivalent amount may escape 

 absorption in the water-absorber following the soda-lime container. 

 With two air-driers used in series, the second one, even under the most 

 exacting conditions with muscular work, rarely gains over 0.1 gm. 



Physiological influences upon the measurement of the respiratory 

 exchange. Aside from the sources of error in the actual manipulation 

 of the apparatus, there is also the possibility of physiological influences. 

 When the subject is inside the chamber, all of the carbon dioxide elimi- 

 nated and all of the oxygen consumed are measured; with the subject 

 on the respiration apparatus, the assumption must be made that the 

 content of the respiratory tract of the man is the same at the end of 

 the experiment as at the beginning that is, the experiment must be 

 begun and ended in such a manner that the subject has in his lungs 

 exactly the same amount of air at both periods. The end of a normal 

 expiration has always been used for the beginning of the experiment. 

 This assumes that the man breathes always to the same depth and that 

 the air left in the lungs is the same in volume at the end of each expira- 

 tion. If the subject begins the experiment with deep breathing and 

 ends the experiment with shallow breathing, it is quite possible that 

 there may be a difference in the total volume of the lungs ; in that case, 

 the determination of the oxygen consumption will be in error. Later 

 experiments with the more recent type of this apparatus have shown 

 that in the majority of experiments the subjects breathed so regularly 

 that the assumption that there was the same amount of air in the 

 lungs at the beginning and end of the experiments is justifiable. There 

 are exceptions to this, however, and it is quite possible that many 

 irregularities which have been obtained with the earlier form of the 

 respiration apparatus may be due to this fact. As a control on the 

 regularity of the breathing, a pneumograph was usually placed around 

 the chest and a graphic record secured. It is very difficult, however, 



