190 PRACTICAL PHYSIOLOGY 



read and the barometric pressure. Potash solution 20 per cent, is then 

 introduced into the eudiometer by means of a pipette provided with a 

 bent end. The carbon dioxide is thus absorbed and the difference in 

 volume read. Pyrogallic acid is then introduced and the oxygen 

 absorbed. The remainder is nitrogen. The temperature of the water 

 jacket is kept constant by adding cold water during the estimation. 

 To correct the volume of gas to and 760 mm. the following formula 

 is employed : y H -/ 



l+t. 0-00367 



where H = the observed pressure, /the tension of aqueous vapour at 

 the observed temperature /. The value of 1 +t. 0-00367 and of / are 

 obtained from tables (cf. Button's Volumetric Analysis). 



CHAPTER L. 

 THE OXYGEN CAPACITY OF BLOOD. 



The Ferricyanide Method of Determining the Oxygen Capacity of 

 Blood. Haldane has introduced a simple method of determining the 

 oxygen in combination with the haemoglobin of the blood. It depends 

 upon the fact that the combined oxygen is liberated rapidly and com- 

 pletely on the addition of a solution of potassium ferricyanide to laked 

 blood. The gas can be easily collected and measured with apparatus 

 similar to that of Dupre for the determination of urea in urine. 



The apparatus used by Haldane is shown in Fig. 187. 



The process is conducted in the following way : 20 c.c. of oxalated 

 or defibrinated blood, thoroughly saturated with air, are measured from 

 a pipette into the bottle A. To this are added 30 c.c. of a weak 

 solution of ammonia made from ordinary strong ammonia solution, 

 sp. gr. 0'88, by diluting with distilled water to -^jo^h- The ammonia 

 prevents the evolution of carbon dioxide and the distilled water lakes 

 the corpuscles. The mixture is thoroughly shaken to complete the 

 laking. Into the tube B are placed 4 c.c. of a freshly saturated solution 

 of potassium ferricyanide. The rubber cork is inserted into the bottle 

 A and the water in the burette is brought to a level close to the top by 

 opening the tap and raising the levelling tube. The tap is closed and 

 the reading of the burette taken. The water gauge attached to the 

 temperature and pressure control tube is adjusted by sliding the rubber 

 tubing backwards or forwards on the glass tube D. 



The bottle A is tilted so that the ferricyanide in B escapes and the 

 mixture is shaken until the evolution of gas has ceased. If the pressure 



