GENERAL DESCRIPTION OF RESPIRATION APPARATUS. 67 



position, while the motor, the controlling rheostat, and the blower are sup- 

 ported by the legs near the floor. The two rubber pipes leading from the 

 table can be used to connect the apparatus either with the bed or chair 

 calorimeter. In fig. 4 the apparatus is shown connected with the bed calo- 

 rimeter, but just above the lowest point of the rubber tubing can be seen in 

 the rear the coupling for one of the pipes leading from the chair calo- 

 rimeter. The other is immediately below and to the left of it. 



OXYGEN SUPPLY. 



The residual air inside of the chamber amounts to some 1,300 liters and 

 contains about 250 liters of oxygen. Consequently it can be seen that in 

 an 8-hour experiment the subject could easily live during the entire time 

 upon the amount of oxygen already present in the residual air. It has been 

 repeatedly shown that until the per cent of oxygen falls to about 11, or 

 about one-half normal, there is no disturbance in the respiratory exchange 

 and therefore about 125 liters of oxygen would be available for respiration 

 even if no oxygen were admitted. Inasmuch as the subject when at rest 

 uses not far from 11 to 15 liters per hour, the amount originally present 

 in the chamber would easily suffice for an 8-hour experiment. Moreover, 

 the difficulties attending an accurate gas analysis and particularly the calcu- 

 lation of the total amount of oxygen are such that satisfactory determina- 

 tions of oxygen consumption by this method would be impossible. Further- 

 more, from our previous experience with long-continued experiments of 

 from 10 days to 2 weeks, it has been found that oxygen can be supplied to 

 the system readily and the amount thus supplied determined accurately. 

 Consequently, even in these short experiments, we adhere to the original 

 practice of supplying oxygen to the air and noting the amount thus added. 



The oxygen supply was formerly obtained from small steel cylinders of 

 the highly compressed gas. This gas was made by the calcium-manganate 

 method and represented a high degree of purity for commercial oxygen. 

 More recently we have been using oxygen of great purity made from liquid 

 air. Inasmuch as this oxygen is very pure and much less expensive than 

 the chemically-prepared oxygen, extensive provisions have been made for 

 its continued use. Instead of using small cylinders containing 10 cubic 

 feet and attaching thereto purifying devices in the shape of soda-lime 

 U -tubes and a sulphuric-acid drying- tube, we now use large cylinders and 

 we have found that the oxygen from liquid air is practically free from 

 carbon dioxide and water-vapor, the quantities present being wholly negli- 

 gible in experiments such as these. Consequently, no purifying attachments 

 are considered necessary and the oxygen is delivered directly from the 

 cylinder. The cylinders, containing 100 cubic feet (2,830 liters), under 

 a pressure of 120 atmospheres, are provided with well-closing valves and 

 weigh when fully charged 57 kilograms. 



