30 COMPARISONS OF RESPIRATORY EXCHANGE. 



of a circle which contains a column or drop of petroleum. The move- 

 ment of the petroleum along the arc of the circle for a few degrees is a 

 very delicate measure of pressure. At the beginning of an experiment 

 the tension equalizer is filled until a slight pressure is shown on the 

 manometer; at the close of the experiment the tension equalizer may 

 readily be brought to the same pressure. 



Method of determining the carbon-dioxide excretion. — The general 

 method for determining the carbon-dioxide excretion is by weight. 

 As has already been pointed out, during an experiment the carbon diox- 

 ide is absorbed completely from the air-current as soon as it reaches 

 the soda-lime container. At the close of the experiment, however, 

 there is carbon dioxide in the air between the mouthpiece or connection 

 to the subject and the carbon-dioxide absorber and it is necessary to 

 sweep this out by continuing the ventilation for about half a minute 

 after an experiment is over in order to absorb completely all of this 

 carbon dioxide. The soda-lime container and its accompanying 

 water-absorber are weighed together before the experiment and again 

 after the experiment, the increase in weight representing the amount 

 of carbon dioxide exhaled by the subject. The weight can then be 

 converted to volume by the factor representing the relation between 

 the weight of carbon dioxide and the volume. The volume per minute 

 may be calculated from the length of the experimental period and the 

 volume exhaled. 



Method of determining the oxygen consumption. — The principle of 

 the determination of the oxygen consumption by means of this appa- 

 ratus has been briefly pointed out in an earlier part of the description. 

 It involves several factors. In the first place, the volume of the appa- 

 ratus must be the same at the beginning as at the end, and this is 

 obtained by admitting air or oxygen into the apparatus before the ex- 

 periment until a slight pressure is reached, as shown by the petroleum 

 manometer. Then at the end of the experiment the same process is 

 repeated, care being taken to have the pressure exactly the same as at 

 the beginning. The other requirement is that the volume of the 

 respiratory tract of the subject be the same at the beginning as at the 

 end. In order to have this true, the experiment is begun at a point in 

 the respiratory cycle which is apparently a constant one, the end of 

 a normal expiration being taken. Numerous observations made in 

 this laboratory with the pneumograph around the chest or abdomen 

 seem to indicate that when the subject is breathing quietly, at rest, 

 the subject empties the respiratory tract to about the same point each 

 time. In practice with the respiration apparatus, therefore, it has 

 been customary to begin the experimental period by turning the three- 

 way valve exactly at the end of a normal expiration and to end the 

 period in the same manner. Having made certain of these two con- 

 ditions, the amount of oxygen admitted into the apparatus from the 



