230 COMPARISONS OF RESPIRATORY EXCHANGE. 



leaks in the apparatus and corrections for leaks may be taken as evi- 

 dence of poor technique. 



Roily has also added another modification of the Benedict method 

 which he considers necessary, i. e., that of equalizing the pressure 

 throughout the apparatus in order to know the true volume at the be- 

 ginning and end of the experimental period. Tests made with the 

 Benedict apparatus in this laboratory have shown that this is entirely 

 unnecessary. If a reading is taken of the spirometer bell before the 

 motor is started and the ventilating current is kept in motion for 15 

 minutes or longer, it will be found that the bell returns to the same 

 position after the motor is stopped as that previous to the beginning of 

 the test. Furthermore, if the movement of the drum is recorded 

 during the 15 minutes that the ventilating current is in motion, after 

 the drum has again settled into position it will be found that the record 

 is a straight line and that there is no change in volume. The equaliza- 

 tion of pressure in the apparatus is wholly unnecessarj^ if the apparatus 

 is constructed according to the design given by Benedict. If the 

 method is employed of filling the spirometer bell to the same point 

 while the apparatus is running, it is necessary to have an electric 

 current which is very constant, otherwise there may be differences in 

 compression in the apparatus due to slight differences in speed. 



While the Benedict apparatus has many advantages, it also has cer- 

 tain disadvantages, previously pointed out by Benedict 1 , which require 

 special care in the technique to overcome. As with all apparatus of 

 the closed-circuit type, the slightest leak vitiates the results. Ex- 

 tremely small leaks may occur so small as to escape detection and 

 even a small leak may change the relation between the carbon-dioxide 

 production and oxygen consumption from a probable figure to one 

 which is wholly improbable; unless it is known absolutely that a leak 

 has occurred, one is in grave doubt as to the necessity of rejecting the 

 figure obtained. If, for example, the respiratory quotients for the 

 successive periods of an experiment have been uniformly 0.85, and the 

 quotient drops in a subsequent period to 0.75 without an apparent 

 cause, the logical inference would be that the change is due to a leak, 

 and yet there may be no proof of it. This uncertainty regarding the 

 occurrence of a leak makes it questionable to assume one. In a study 

 made of the effect of a no-carbohydrate diet upon the respiratory 

 exchange, it was found that in one period of an experiment there was 

 a tendency for the respiratory quotient to be higher than that which 

 would be expected. The pneumatic nosepieces were used in this ex- 

 periment with a good subject, but upon inflating them to a somewhat 

 greater distention, so that they fitted the nose still more closely, results 

 were obtained which were more nearly in accord with earlier periods. 



Benedict, Am. Journ. Physiol., 1909, 24, p. 345. 



