16 COMPARISONS OF RESPIRATORY EXCHANGE. 



sodium-bicarbonate can and the chamber. The air enters the cham- 

 ber at the top, at a point near the front end. The average rate of 

 ventilation is about 40 liters per minute; thus there is a wind move- 

 ment in the chamber of about 1.6 mm. per second. A thorough mix- 

 ture of the air in the chamber is brought about by the use of an elec- 

 tric fan situated at the rear upper portion of the apparatus. 



The water- vapor given off by the subject is removed by passing the 

 circulating air-current through sulphuric acid contained in a porcelain 

 vessel, the general shape and construction of which are shown in figure 1 

 (see 1 and 2). The air enters at the top of the vessel and is broken 

 up in its passage through the acid by means of three concentric circles 

 of openings; it then leaves at the top. Three liters of the strongest 

 commercial sulphuric acid are used, the container and acid weighing 

 about 18 kg. 



The carbon dioxide is removed by passing the air through slightly 

 moist soda-lime. This is packed loosely in silver-plated brass cylin- 

 drical cans. (See K, fig. 1.) As the dry air in passing through the 

 moist soda-lime absorbs water, another sulphuric-acid container, 2, 

 is attached to the exit end of the carbon-dioxide absorber to absorb 

 the water- vapor coming from the soda-lime. 



All three pieces of apparatus are provided with couplings so that 

 they may be detached and weighed, the weighings being made on a 

 Sauter balance with an accuracy of 0.1 gm. A duplicate set of absor- 

 bers is provided and valves are placed at the ends of each series. By 

 closing the valves attached to one set and opening those attached to the 

 duplicate absorbers, the ventilating current may be deflected from one 

 set to the other. This permits the division of the experiment into 

 periods. 



The supply of oxygen is maintained by automatic admission from a 

 weighed cylinder. This cylinder contains when full about 100 cubic 

 feet (2,800 liters) and weighs about 50 kg. It is hung on one arm of a 

 large Sauter balance and can be weighed with an accuracy of 0.1 gm. 

 The admission of oxygen is regulated by the change in volume of the 

 air in the apparatus. An opening in the side of the chamber is con- 

 nected with a spirometer, 1 this spirometer being simply a light copper 

 cylinder which is counterpoised and suspended in water. As the water- 

 vapor and carbon dioxide are removed, the volume of air in the appa- 

 ratus diminishes and the bell gradually sinks; oxygen is admitted from 

 time to time to keep the bell at a convenient height. In actual prac- 

 tice with the apparatus, the admission is accomplished automatically 

 by an electrical arrangement. When the bell drops to a certain point, 

 an electric circuit is closed. In this electric circuit is an electro- 



1 Formerly another type of tension equalizer was used in which a rubber bathing-cap was 

 attached to the upper end of a tin can. The details of its construction and use are given in 

 Benedict and Carpenter, Carnegie Inst. Wash. Pub. No. 123, 1910, p. 71. 



