CALCULATION OF RESULTS. 89 



calculation has been nsed with success in connection with the large chamber 

 and particularly for experiments of short duration. It has also been intro- 

 duced with great success in a portable type of apparatus described else- 

 where.* Under these conditions, therefore, it is unnecessary to make any 

 correction on the residual volume of nitrogen as calculated at the beginning 

 of the experiment. When a direct comparison of the calculated residual 

 amount of oxygen present is to be made upon determinations made with a 

 gas-analysis apparatus the earlier and much more complicated method of 

 calculation must be employed. 



CRITICISM OF THE METHOD OF CALCULATING THE VOLUME OF OXYGEN. 



Since the ventilating air-current has a confined volume, in which there 

 are constantly changing percentages of carbon dioxide, oxygen, and water- 

 vapor, it is important to note that the nitrogen present in the apparatus 

 when the apparatus is sealed remains unchanged throughout the whole 

 experiment, save for the small amounts added with the commercial oxygen — 

 amounts well known and for which definite corrections can be made. Con- 

 sequently, in order to find the amount of oxygen present in the residual air 

 at any time it is only necessary to determine the amounts of carbon dioxide 

 and water-vapor and, from these two factors and from the known volume of 

 nitrogen present, it is possible to compute the total volume of oxygen after 

 calculating the total absolute volume of air in the chamber at any given 

 time. 



While the apparent volume of the air remains constant throughout the 

 whole experiment, by the conditions of the experiment itself the absolute 

 amount may change considerably, owing primarily to the fluctuations in 

 barometric pressure and secondarily to slight fluctuations in the tempera- 

 ture of the air inside of the chamber. Although the attempt is made on 

 the part of the observers to arbitrarily control the temperature of this air 

 to within a few hundredths of a degree, at times the subject may inadvert- 

 ently move his body about in the chair just a few moments before the end 

 of the period and thus temporarily cause an increased expansion of the air. 

 The apparatus is, in a word, a large air-thermometer, inside the bulb of 

 which the subject is sitting. If the whole system were inclosed in rigid 

 walls there would be from time to time noticeable changes in pressure on 

 the system due to variations in the absolute volume, but by means of the 

 tension-equalizer these fluctuations in pressure are avoided. 



The same difficulties pertain here which were experienced with the earlier 

 type of apparatus in determining the average temperature of the volume of 

 air inside of the chamber. We have on the one hand the warm surface of 

 the man's body, averaging not far from 32° C. On the other hand we have 



* Francis G. Benedict : An apparatus for studying the respiratory exchange. 

 American Journal of Physiology, vol. 24, p. 368. (1909.) 



