THE RESPIRATION APPARATUS. 39 



nitrogen in percentages below 2.50, the calculation of the percentage 

 composition by weight can be very accurately made by multiplying the 

 volume percentage by the factor 0.882. For all volume percentages 

 between 2.50 and 3.80 the factor in use is 0.883. ^ or higher percent- 

 ages the factor is increased o.ooi for every 0.95 per cent of increase in 

 the nitrogen content. At present we rely wholly on this method. 



In order to minimize the actual amount of work during the progress 

 of an experiment, the cylinders of oxygen are analyzed and the per- 

 centage composition by volume aud weight determined several days 

 before the experiment begins. 



THE TENSION EQUALIZERS. 



The volume of the air inside the closed circuit is subject to continued 

 fluctuations as a result of changes in temperature, barometric pressure, 

 oxygen consumption, and water and carbon-dioxide absorption. In 

 order to maintain at all times atmospheric pressure inside the respiration 

 apparatus, and thereby reduce to a minimum the danger of rupturing 

 the comparatively thin chamber walls and the liability of leakages 

 throughout the system, a compensating device was arranged, which is 

 shown in figure 17. 



The device consists of two pans, connected with the main air-pipe 

 and covered at the top with flexible rubber diaphragms, which by their 

 expansion allow for considerable variation in the total volume of the 

 circuit. The diaphragms are made of pure gum, molded to fit the inner 

 surface of the pan, and so formed as to lap over the edge. A stout rub- 

 ber band is snapped over the edge of the pan so as to hold the edge of 

 the rubber diaphragm closely against the rim of the pan, making a 

 very tight closure. A hard rubber disk of a diameter a few millimeters 

 less than that of the inside bottom of the pan is cemented to the top of 

 the diaphragm by means of fish glue. Three holes at equal distance 

 in the periphery of this disk provide for three loops of wire which meet 

 at a point directly above the center of the disk and are there fastened 

 to a small ring. The weight of the diaphragm aud disk, distributed 

 as it is over the whole system, exerts comparatively slight pressure. 

 In order to eliminate pressure entirely, however, the weight is counter- 

 balanced by suspending the diaphragm on a fine, flexible steel wire 

 running over the rim of a bicycle wheel, the edge of which is so placed 

 that the wire hangs perfectly plumb and without lateral strain or pull. 

 On the opposite side of the rim of the wheel a similar flexible wire is 

 attached, the lower end of which is weighted with a counterpoise con- 

 taining shot. The bicycle wheel, having ball bearings, is extremely 

 sensitive, and it is possible to adjust the weight of shot in the counter- 



