42 COMPARISONS OF RESPIRATORY EXCHANGE. 



of the changes in weight of the second bottle; when it has gained 10 gm. 

 of water- vapor it is rejected and another substituted. The method 

 of insuring the efficiency of the soda-lime containers has already been 

 given in the description of the tension-equalizer unit, and applies to 

 this apparatus. The efficiency of the Williams bottle following the 

 carbon-dioxide absorber is also safeguarded as described on p. 32. 



Before each experiment the three-way valve should be taken out, 

 thorough!}^ sterilized, and lubricated with vaseline in such a manner 

 that it will turn easily without danger of a leak. The mouthpiece, 

 moistener, and nosepieces should also be sterilized before each experi- 

 ment and again immediately after the experiment. 



The bell of the spirometer should be examined occasionally to make 

 sure that it does not touch the copper walls of the bath. It should 

 hang perfectly vertical and move up and down midway between the 

 two cylinders. The ventilation adder contact should likewise be 

 inspected before the experiment is begun to find if it works properly 

 when the wheel is placed at zero. 



Calibration of the bell of the spirometer. The records of the movement 

 of the spirometer bell up or down are used in the measurement of the 

 oxygen consumption and also in the measurement of the volume of 

 respiration, each millimeter representing a certain quantitative rela- 

 tion of volume (usually 21 to 23 c.c.). This value may be ascertained 

 in several different ways. It may be calculated from the height and 

 the diameter of the spirometer bell by the usual method of calculating 

 the volume of a cylinder. This assumes that the bell is a perfect 

 cylinder, with no irregularities in any part. Another method is to 

 invert the bell of the spirometer, fill it with water at a definite tempera- 

 ture, and compare the weights obtained before and after filling it. In 

 using this method the bottom of the cylinder must be well supported 

 to prevent bulging; the cylinder must also be absolutely level, other- 

 wise it is impossible to fill the cylinder to its full capacity. A third 

 method of calibrating the bell, and the most common in this laboratory, 

 is by the admission of a definite quantity of air or oxygen through a 

 Bohr meter. A description of this meter is given in connection with 

 the description of the method of admitting oxygen to the apparatus 

 (see page 47). The spirometer bell is pushed down to the lowest 

 possible limit and a reading on a millimeter scale is taken. Air or 

 oxygen is then passed through the meter into the bell of the spirometer; 

 when the bell has risen to its full height, the oxygen or air is shut off. 

 From the reading of the meter, the factor of the meter, and the num- 

 ber of millimeters to which the bell has risen, the value per millimeter 

 may be calculated. A correction should be made for temperature 

 if the temperatures of the meter and the spirometer are markedly 

 different. 



