CHARACTER OF KATABOLISM 73 
Errors Incidental to the Determination op the Carbon-dioxide Production. 
In the type of apparatus here used the carbon-dioxide production is 
determined by absorbing the gas in suitable containers filled with soda-lime. 
No volumetric gas-analyses, no aliquoting of samples, and no records of the 
variations in the barometer and the pressure are required. The determination 
of the carbon dioxide in the air-current thus becomes a method of gravimetric 
analysis. There are, however, several possible sources of error. 
The first to be considered is an incomplete absorption of the carbon 
dioxide due to the inefficiency of the soda-lime. To provide for the absorption 
of any excess of carbon dioxide the air was swept through the system for 
several minutes at the end of each experimental period, the air-current pass- 
ing through the soda-lime two, three or even four times. Furthermore, a 
test of the air leaving the soda-lime bottles was frequently made by passing a 
small sample of it through barium hydroxide, the complete absorption of the 
carbon dioxide being shown by the absence of turbidity. We have also found 
an ann oying source of error in the fact that when the absorbers are weighed, 
the rubber gaskets used to secure tight closure between the different recep- 
tacles are occasionally left in the couplings. Each weighing of the soda-lime 
bottles was therefore checked by a second person and the absence of the 
gasket noted on the record sheet when the record of the weight was made, 
thus eliminating the possibility of error from this cause. 
Another way in which the determinations of the carbon dioxide may be 
affected is by the inefficiency of the sulphuric acid in the air-drying bottle. 
If, through carelessness, the sulphuric acid in this bottle is allowed to absorb 
more than 10 grams of the water taken up in the passage of the air through 
the soda-lime, the absorption will not be quantitative and the determined 
amount of carbon dioxide will be lower than that actually produced. To pro- 
vide against this error, it was the custom to have the two soda-lime bottles and 
the air-drying bottle removed at the end of each day's experiment, repeated 
tests having shown that the limit of absorption, namely, 10 grams, was never 
reached under these conditions. In view of these precautions, we have every 
confidence in the measurements of the carbon-dioxide production. 
Errors Incidental to the Determination op the Oxygen Consumption. 
The determination of the oxygen consumption can be made either 
gravimetrically, as was originally designed for this apparatus, by weighing 
the oxygen cylinder before and after the experiment and noting the amount 
of oxygen added to the air-current, or by the more convenient and recently 
adopted method of measuring the amount of oxygen by means of a carefully 
calibrated Bohr gas-meter immersed in water. This method of measuring 
the oxygen consumption depends upon the fact that the subject uses out of 
the air in the system a certain amount of oxygen which must be replaced 
by pure oxygen or by a known volume of some other gas. If the oxygen 
in the system is not allowed to fall below 10 or 11 per cent, even pure nitrogen 
might be used to replace the oxygen absorbed and bring the air to the original 
volume, since it has been shown that the metabolism is normal above this 
oxygen percentage. For this replacement, however, we ordinarily use oxygen 
« More recent tests kindly carried out by Mr. T. M. Carpenter have shown conclusively that no appreciable 
amount of water escaped absorption. 
