42 A RESPIRATION CALORIMETER. 



position, obviously gave the entire content of the pan when filled. In 

 calibrating the other pan the procedure was identical with that outlined 

 above, save that the pan was calibrated for intermediate positions of 

 the diaphragm . These were determined by means of a pointer attached 

 to the bicycle wheel. As the diaphragm rises or falls the wheel turns, 

 and the pointer travels over a graduated arc reading to millimeters. 

 In the calibration, as the dial on the meter passed each half-liter mark, 

 the reading of this pointer was taken. The actual volume of air was 

 then determined for each point on the scale. These points were subse- 

 quently plotted on a curve, and as a result it is only necessary to adjust 

 one pan so that it is either full or empty, and to read the pointer on 

 the other in order to estimate very exactly, that is, probably within o. i 

 liter, the actual volume of air in the two pans. 



In the course of a year's experimenting the sulphuric acid in the 

 enamel dishes inside the pans will gradually absorb moisture and con- 

 sequently increase in volume. This increase in volume is, however, 

 negligible. 



POSSIBILITY OF NOXIOUS GASES IN THE SYSTEM. 



An anticipated objection to the use of the closed circuit was the pos- 

 sibility of introducing noxious gases into the apparatus. It is readily 

 conceivable that relatively small amounts of sulphuric-acid vapor, or 

 mercury vapor, for example, would be extremely injurious to the health 

 of the subject. Since the air current comes in contact with sulphuric 

 acid in the absorbers and to a less extent with mercury vapor in the 

 valves, it was especially necessary to determine carefully the extent to 

 which these substances might be carried into the system. 



Acid fumes carried over by air current. Reference has already been 

 made to the pumice-stone traps on the exit tube leading from the water- 

 absorbers. These serve to diminish the possibility that acid will be 

 carried along mechanically. As an additional safeguard, a layer of 

 cotton, kept in place by a wire- gauze thimble such as is used in the 

 carbon-dioxide absorbers, is inserted in the rubber tube connecting the 

 last water- absorber with the air-pipe leading back to the chamber. As 

 a result of practical experience it has been found that this cotton serves 

 to retain any acid fumes in the air current. 



Mercury vapor in the air. Owing to the marked susceptibility of 

 certain individuals to the toxic properties of minute quantities of 

 mercury vapor, care was necessary to obviate all danger of poisoning. 

 As has been described, the mercury valve (fig. 10) depends upon mer- 

 cury to effect a tight closure. To be sure, the construction of this valve 

 is such that when it is open and air is passing through it all of the 



