244 COMPARISONS OF RESPIRATORY EXCHANGE. 



the smaller spirometer, but only to 0.1 or 0.2 liter with the 200-liter 

 spirometer. This fact should be taken into consideration in experi- 

 ments with the Tissot method. For example, in making experiments 

 in which the periods are very short and the volume of air to be collected 

 is not more than 50 liters, it is advisable to use the smaller spirometer, 

 as more accurate readings may be obtained with it. In all but five 

 of the comparison experiments in which the Tissot method was used 

 the 200-liter spirometer was employed, the volume of air collected 

 usually being 75 to 100 liters. 



As the spirometer bell rises out of the water, some moisture adheres 

 to the sides. This has a certain cooling effect, at least upon the outside, 

 and may affect the volume of air inside the bell. To determine this 

 possible influence, a 100-liter spirometer was filled as quickly as possi- 

 ble with room air and readings of the temperature of the air were taken 

 every minute in the usual way, also readings of a thermometer hung as 

 closely as possible to the water-level and to the side of the bell. The 

 thermometer near the water-level showed a marked cooling effect after 

 the bell had come to rest and the thermometer in the opening at the top 

 of the spirometer bell indicated a simultaneous cooling effect upon the 

 volume of air inside. By using the readings of the latter thermometer 

 and calculating the volume of air in the spirometer to C. and 760 

 mm. pressure, it was found that the variations in volume due to this 

 cooling were less than 0.2 liter with a volume of 70 liters. Since this 

 is less than 0.3 per cent, the possible error due to cooling must be 

 very small, especially as the variation noted is also subject to possible 

 errors in the reading of the volume of air and of the temperature. 



The errors of the second class, i. e., those affecting the sampling and 

 analysis of the air collected in the spirometer, have occupied the atten- 

 tion of a great many observers. Durig 1 has pointed out that there is a 

 possibility of stratification in collecting expired air by this method and 

 that such stratification may cause a considerable error when a large 

 volume is sampled. To study this point and also to test the general 

 accuracy of the Tissot method in the measurement of the carbon- 

 dioxide content of expired air, a series of experiments was carried out 

 in the following manner: 



A pair of Tissot valves was attached to the hand spirometer 2 by 

 means of a glass tee. A small opening was made in the side of the 

 glass tee and carbon dioxide was introduced from a cylinder of the 

 compressed gas, the carbon dioxide passing through a 1-liter Bohr meter. 

 The meter was immersed in a tank, as for the measurement of oxygen 

 by the Benedict method. The outgoing valve was connected to a 

 200-liter Tissot spirometer. By raising and lowering the bell of the 

 hand spirometer and drawing carbon dioxide intermittently through 



'Durig, Archiv f . Anatomie und Physiologic. Phyeiologische Abteilung, 1903, p. 219. 

 *See p. 252. 



