808 CMA>fGE OP COMPOSITION' OF ALVEOLAR AIR, 



Positive pressures. — In the following Table are given the per- 

 centages of carbon dioxide found in the alveolar air after holding 

 the breath under various pressures in excess of that of the 

 atmosphere. The corresponding percentages of carbon dioxide 

 reached, when the breath is held under normal pressure, are 

 given for comparison. 



When the breath is held under pressures differing from that 

 of the atmosphere, it is found to be rather difficult to note the 

 time to within a second. The subject is obliged to watch the 

 manometer as well as to observe the time. The figures in the 

 accompanying Tables show that variations of several seconds 

 occur in the periods of holding the breath, as determined from 

 the graphic records. The percentages of alveolar carbon dioxide 

 found, therefore, cannot be compared directly with one another, 

 and it has been necessary to reduce the results to a common 

 period. In the last column of the Tables, the alveolar percent- 

 ages of carbon dioxide are given, reduced to a period of holding 

 the breath of 20 seconds. In the case of Table vi., this reduc- 

 tion has been made from the data given in the average figures 

 in Table i. From these figures it will be seen that, between the 

 21st and 26th seconds of holding the breath, the alveolar carbon 

 dioxide rises at the rate of 0-025% per second. During a period 

 of this length, the rise is very nearly uniform, as is shown by 

 curve A (Text-fig.l). For each second for which the breath was 

 held longer than 20 seconds, 0025 has been subtracted, therefore, 

 from the percentage of carbon dioxide found. 



