408 RESPIRATION 



acid, and its pressure measured. When certain corrections are made, it 

 is then possible to estimate either the total oxygen and total CO 2 , or the 

 combined oxygen and combined CO 2 in the blood. The gas is measured 

 by the increase of pressure at constant volume, and not by the increase 

 of volume at constant pressure. Theoretically, either method is correct, 

 in accordance with Boyle's Law ; but as Barcroft required a method for 

 dealing with very small quantities of blood, and a very delicate pressure- 

 gauge was needed in any case, it seemed simpler to graduate the pres- 

 sure gauge in millimeters, and keep the gas at constant volume, retain- 

 ing, however, the control vessel, as in the original form of apparatus. 

 I therefore designed the apparatus as it was originally figured in our 

 paper, and the tests we made gave very satisfactory results so far as 

 they went. 



One defect of the apparatus described in the previous section is that 

 a considerable time is needed to reach temperature equilibrium and to 

 shake out all the extra free oxygen from the blood solution. The latter 

 defect would apply still more to an apparatus in which CO 2 had to be 

 shaken out. In the new apparatus the volume of liquid was therefore 

 greatly diminished, and the relative volume of air to blood solution 

 greatly increased; and this was also rendered advisable owing to the 

 fact that nearly as much CO 2 remains in solution in the liquid as is 

 present in an equal volume of air. The increased volume of air had, 

 however, the disadvantages, first that the pressure of ammonia in the 

 air introduced an appreciable source of error, and secondly that much 

 more care was needed as to temperature equilibrium in the blood vessel 

 and control vessel. A further source of error was slight variation in 

 capillarity at different levels in the gauges of the blood vessel and 

 control vessel. In spite of all improvements in this apparatus and the 

 methods of using it, there appears to be a range of error with it of at 

 least 2 per cent of the quantity measured, even when the error due to 

 ammonia vapor is completely eliminated. 



The apparatus was rendered much more convenient, though also less 

 easy to make or repair, by Brodie. 8 It was also simplified by Barcroft; 

 who named his modification the "differential" apparatus. 9 Barcroft con- 

 nects the gauges of the blood vessel and control vessel, so that there is 

 only one manometer instead of two, and estimates the gas given off from 

 the readings of this compound gauge. With this construction the ap- 

 paratus works at neither constant volume nor constant pressure, so that 

 the gas given off cannot be correctly deduced from the mere readings of 

 the gauges. He therefore calibrates the apparatus empirically with the 



8 Brodie, Journ. of Physiol., XXXIX, p. 391, 1910. 



9 Described fully in Barcroft's book, The Respiratory Functions of the Blood. 



