40 A RESPIRATION CALORIMETER. 



poise so as to compensate for practically all the weight of the rubber 

 diaphragm. It is evident that the higher the diaphragm is raised the 

 greater the proportion of its weight that is suspended, as more of the 

 rubber diaphragm is then suspended from the central rubber disk and 

 less from the edge of the pan, though as a matter of fact the slight 

 variations in weight, amounting to but a few grams for the different 

 positions of the diaphragm, are distributed over so large an area that 

 it is impossible to note any difference in fluctuation of the water man- 

 ometer. In practice the counterpoise is so adjusted that the rubber 

 diaphragm remains in a position about half way between the top and 

 the bottom when all connections are open. 



Under these conditions it is assumed that the pressure on the whole 

 respiration system, when the blower is not in operation, is atmospheric, 

 except in the carbon-dioxide and water absorbers, as explained on page 

 73. The resistance of the length of pipe between the pans and the 

 respiration chamber is sufficient to cause the pans to rise rather than 

 fall when the blower is running and the pipe is open, but under all 

 conditions of passing air through the system it has been found practi- 

 cally impossible to detect any differences in pressure in the chamber 

 proper, since the pans so perfectly compensate for variations in baro- 

 metric pressure and other changes in volume. 



It will be noticed that the pans are connected with the air-pipe be- 

 tween the pump and the respiration chamber. The air in the pans 

 has therefore been freed from carbon dioxide and water. There is as 

 yet no evidence to indicate that carbon dioxide enters through the rub- 

 ber diaphragm in measurable amounts. It has been found, however, 

 that appreciable quantities of water vapor may be admitted into the 

 system in this way. To guard against this an enameled- ware dish, 

 half filled with concentrated sulphuric acid, is placed in the bottom of 

 each pan. As the result of a number of tests it was found that when 

 this precaution was taken no weighable quantities of water vapor enter 

 the closed air-circuit through the pans, whatever diffuses through the 

 rubber being apparently retained by the sulphuric acid. To prevent 

 the rubber diaphragm from coming into contact with the acid, the dish 

 containing the latter is covered with a wire gauze. 



The volume of air in each pan consists of two portions, one of which 

 is constant, the other variable. The constant volume comprises that 

 portion contained when the rubber diaphragm is at its lowest point, 

 i. <?., resting on the gauze cover to the sulphuric-acid dish. The fluc- 

 tuating volume is limited by the highest position of the diaphragm. 

 The two pans allow for fluctuations in volume of about 29 liters. 



