690 OSTERHOUT RESPIRATORY APPARATUS 



of this change is assumed to be identical with that of the excretion 

 of carbon dioxide by the organism. 



In calibrating the apparatus various means were used to produce 

 a constant and measurable flow of carbon dioxide as a substitute 

 for that from the organism. About 100 cc. of a normal solution of 

 sulfuric acid were poured into the chamber in which the organisms 

 had been kept, and into this were dripped at a constant rate solutions 

 of sodium bicarbonate of different but known concentrations. 

 The rates of change in the indicator did not correspond to the cal- 

 culated rates of liberation of carbon dioxide from the solutions, and 

 it soon becomes apparent that all the carbon dioxide produced was 

 not liberated but that part of it remained behind in the solution. 

 The dripping of weak acid on dry sodium bicarbonate or on marble 

 also failed to yield consistent results. 



The method of calibration finally adopted was that of making 

 mixtures of carbon dioxide and atmospheric air and of introducing 

 them at a known rate into the apparatus in place of the organism. 

 Into a narrow graduated glass receiver filled with water carbon diox- 

 ide was bubbled till a given volume at atmospheric pressure was 

 attained, whereupon, the remainder of the receiver was quickly filled 

 mth atmospheric air and the whole closed. This operation was 

 carried on over a water bath so quickly that the solution of the 

 carbon dioxide in the water was reduced to a negligible amount. 

 Mixtures of gases containing 0.4, 1, 2, and 4 per cent of carbon 

 dioxide were used. Each of these mixtures was introduced through 

 a connecting tube into the chamber of the apparatus in which the 

 organism had been kept by running into the gas receiver at a known 

 rate a minute stream of mercury and thus driving out the mixture 

 of gas. As the glass gas receiver was graduated, the amount of gas 

 driven out of it in any given time could be easily read off by observing 

 the levels at which the mercury stood in it as shown by the scale on 

 its wall. Thus a constant and measurable supply of carbon dioxide 

 was substituted for that produced by the organism. In this form 

 of procedure the rate of change in the indicator corresponded very 

 closely to the rate of introduction of carbon dioxide. Since the 

 volume of mixed gases introduced into the apparatus in any test 

 was very small in comparison with the total volume of the apparatus, 



