GENERAL DESCRIPTION OF RESPIRATION APPARATUS. 73 



ANALYSIS OF RESIDUAL AIR. 



The carbon-dioxide production, water-vapor elimination, and oxygen 

 absorption of the subject during 1 or 2 hour periods are recorded in a 

 general way by the amounts of carbon dioxide and water-vapor absorbed 

 by the purifying vessels and the loss of weight of the oxygen cylinder ; but, 

 as a matter of fact, there may be considerable fluctuations in the amounts 

 of carbon dioxide and water-vapor and particularly oxygen in the large 

 volume of residual air inside the chamber. With carbon dioxide and water- 

 vapor this is not as noticeable as with oxygen, for in the 1,300 liters of air 

 in the chamber there are some 250 liters of oxygen, and slight changes in 

 the composition of this air indicate considerable changes in the amount of 

 oxygen. Great changes may also take place in the amounts of carbon 

 dioxide and water-vapor under certain conditions. In some experiments, 

 particularly where there are variations in muscular activity from period to 

 period, there may be a considerable amount of carbon dioxide in the residual 

 air and during the next period, when the muscular activity is decreased, for 

 example, the percentage composition of the air may vary so much as to 

 indicate a distinct fall in the amount of carbon dioxide present. Under 

 ordinary conditions of ventilation during rest experiments the quantity of 

 carbon dioxide present in the residual air is not far from 8 to 10 grams. 

 There are usually present in the air not far from 6 to 9 grams of water- 

 vapor, and hence this residual amount can undergo considerable fluctua- 

 tions. When it is considered that an attempt is made to measure the total 

 amount of carbon dioxide expired in one hour to the fraction of a gram, it 

 is obvious that fluctuations in the composition of residual air must be taken 

 into consideration. 



It is extremely difficult to get a fair sample of air from the chamber. 

 The air entering the chamber is free from water-vapor and carbon dioxide. 

 In the immediate vicinity of the entering air-tube there is air which has a 

 much lower percentage of carbon dioxide and water-vapor than the average, 

 and on the other hand close to the nose and mouth of the subject there is air 

 of a much higher percentage of carbon dioxide and water-vapor than the 

 average. It has been assumed that the composition of the air leaving the 

 chamber represents the average composition of the air in the chamber. 

 This assumption is only in part true, but in rest experiments (and by far 

 the largest number of experiments are rest experiments) the changes in the 

 composition of the residual air are so slow and so small that this assump- 

 tion is safe for all practical purposes. 



Another difficulty presents itself in the matter of determining the amount 

 of carbon dioxide and water-vapor; that is, to make a satisfactory analvsis 

 of air without withdrawing too great a volume from the chamber. The 

 difficulty in analysis is almost wholly confined to the determination of 



