72 CALORIMETERS FOR STUDYING RESPIRATORY EXCHANGE, ETC. 



In most of the experiments made thus far it has been our custom to 

 conduct the supply of fresh oxygen through pet-cock K on the side of the 

 tension-equalizer. This is shown more in detail in fig. 32, in which, also, is 

 shown the interior construction of the can. Owing to the fact that the air 

 inside of this can is much dryer than the room air, we have followed the 

 custom with the earlier apparatus of placing a vessel containing sulphuric 

 acid inside the tension-equalizer, so that any moisture absorbed by the dry 

 air inside the diaphragm may be taken up by the acid and not be carried 

 into the chamber. The air passing through the pipe to the calorimeter is, it 

 must be remembered, absolutely dry and hence there are the best conditions 

 for the passage of moisture from the outside air through the diaphragm 

 to this dry air. Attaching the tension-equalizer directly to the calorimeter 

 obviates the necessity for this drying process and hence the sulphuric-acid 

 vessel has been discarded. 



The valve H (fig. 25) is used to cut off the tension-equalizer completely 

 from the rest of the system at the exact moment of the end of the experi- 

 mental period. After the motor has been stopped and the slight amount of 

 air partly compressed in the blower has leaked back into the system, and 

 the whole system is momentarily at equal tension, a process occupying some 

 3 or 4 seconds, the gate-valve H is closed. Oxygen is then admitted from 

 the pet-cock K until there is a definite volume in J as measured by the 

 height to which the diaphragm can rise or a second pet-cock is connected 

 to the can I and a delicate petroleum manometer attached in such a manner 

 that the diaphragm can be filled to exactly the same tension each time. 

 Under these conditions, therefore, the apparent volume of air in the sys- 

 tem, exclusive of the tension-equalizer, is always the same, since it is con- 

 fined by the rigid walls of the calorimeter and the piping. Furthermore, 

 the apparent volume of air in the tension-equalizer is arbitrarily adjusted 

 to be the same amount at the end of each period by closing the valve and 

 introducing oxygen until the tension is the same. 



BAROMETER. 



Eecognizing the importance of measuring very accurately the barometric 

 pressure, or at least its fluctuations, we have installed an accurate barome- 

 ter of the Fortin type, made by Henry J. Green. This is attached to the 

 inner wall of the calorimeter laboratory, and since the calorimeter labora- 

 tory is held at a constant temperature, temperature corrections are unnec- 

 essary, for we have here to deal not so much with the accurate measure- 

 ment of the actual pressure as with the accurate measurement of differences 

 in pressure. For convenience in reading, the ivory needle at the base of the 

 instrument and the meniscus are well illuminated with electric lamps 

 behind a white screen, and a small lamp illuminates the vernier. The 

 barometer can be read to 0.05 millimeter. 



