48 



COMPARISONS OF RESPIRATORY EXCHANGE. 



easily be met. The mechanical correction factor can be obtained by 

 calibration tests, 1 in which a cylinder of oxygen is used, the amount of 

 gas passing through the meter being computed from the loss in weight 

 and from the known chemical composition of the gas. Before the 

 meter is calibrated it should be accurately leveled by means of the 

 leveling screws on the meter and on the board upon which it rests. 

 It should also be filled to the level at which it is to be used, the best 

 level being that indicated by the manufacturers by the lines marked 

 upon the rim. The meter and aquarium jar with the surrounding 

 water should stand long enough before calibration for the whole mass 

 to come into temperature equilibrium, otherwise the temperature of 



Table 6. — Results of independent calibrations of a 1-liter Bohr meter 

 by two operators. 



the bath may not indicate the temperature of the meter. It is also 

 necessary that the cylinder connections be absolutely air-tight. This 

 may be tested by weighing the cylinder at intervals of 15 to 20 minutes; 

 if no change in weight takes place, the connections are tight. The 

 cylinder is then connected to the entrance tube of the moistening 

 apparatus and the gas is passed through at approximately the rate to 

 be used during an experiment. Usually this has been about 4 liters in 

 10 to 15 minutes. The two or three calibrations made in this manner 

 should agree within 0.5 per cent, and the limits of error between two 

 sets of calibrations made by two people on separate days should agree 

 on the average within at least 1 per cent. 



*The method has been described in detail by Benedict, Phys. Review, 1906, 22, p. 294. 



