THE CHEMISTRY OF RESPIRATION 



1193 



The performance of a tonometer may be expressed by the ratio of the surface 

 of blood exposed to the volume of the air used. The ' specific surface ' of an aero- 

 tonometer is represented by - . The specific surface of Pfliiger's 



volume in c.c. 



instrument is only 3-3 and of Bohr's only 5-2. In Krogh's microtonometer the 

 absolute volume of air employed is reduced to a bubble of about 2 mm. in diameter, 

 having a volume of -004 c.c. and a surfaceof 0-125 sq. cm., so that its specific surface 

 is 30. In such a bubble the equalisation of the tensions takes place with an extreme 



FIG. 500. A, Krogh's microtonometer. B, upper part of microtonometer 

 showing capillary tube into which the bubble is returned for measure- 

 ment and analysis. 



rapidity and only a minute quantity of fluid is necessary. The microtonometer 

 consists of the tonometer proper and the apparatus for the microanalysis of the gas 

 bubble. In the latter the measurement of the gas bubble is carried out in a 

 capillary tube, the absorption of carbon dioxide and of oxygen being carried out 

 in the usual way with potash and with pyrogallic acid. The tonometer is repre- 

 sented in Fig. 500. It is kept in a small water -bath at the temperature of the 

 blood to be examined. The tonometer is filled with saline solution and contains 

 the gas bubble 2, which can be drawn up by means of the screw 4 into the narrow 

 graduated tube 3, where its volume is measured. The blood from the artery or 

 vein, the tension of the gas in which we wish to examine, passes by a cannula 

 through the tube 1, and enters the tonometer as a fine jet. It forces its way 

 up above the gas bubble, which is pressed a little down by the current, and 

 kept oscillating with great rapidity, From the tonometer the blood flows back 



