364 



RESPIRATION 



[CR. XXVI. 



potash, and the gas that remains consists of oxygen and nitrogen ; 

 the oxygen is then removed by pyrogallic acid, and the residual gas 



is nitrogen. (2) The chemical method * 

 adopted is as follows : 



When a solution of oxyhsemoglobin 

 is shaken with potassium ferricyanide, 

 it yields the same amount of oxygen 

 to the air as it would if boiled in a 

 vacuum. In much the same way urea 

 when treated with sodium hypobromite 

 yields up all its nitrogen, and the 

 apparatus used for determining the 

 oxygen in blood is very similar to a 

 Dupre's urea apparatus (see Chapter 

 XXXIX.). The blood (5 c.c.) is placed 

 in the large bottle (fig. 293, A) under- 

 neath a layer of dilute ammonia solu- 

 tion (B). The blood is thus protected 

 from the air whilst the apparatus 

 becomes equal in temperature to the 

 bath in which it is placed. The blood 

 is shaken with the ammonia solution, 

 which lakes it thoroughly; the ferri- 

 cyanide solution is then spilt into the 

 laked blood from the tube C, and the 

 oxygen is shaken out of the solution. 

 When the oxygen has been determined 

 the bottle is opened and tartaric acid 

 is placed in the small tube C ; this is 

 subsequently spilt into the mixture of 

 blood, ammonia, and ferricyanide; it 

 liberates the carbonic acid which is also 

 shaken out of the fluid. The carbonic 

 acid does not come completely out, 

 however, and a correction has to be 

 introduced for the quantity which 

 remains in solution. The gas (oxygen 

 or carbonic acid, as the case may be) passes over into the tube 

 D, which was previously filled up to the zero mark with water, 

 and connected to a reservoir (F) ; this would drive water out of F 



* The apparatus figured is one of the earliest, and many modifications have 

 been invented since which are in use in different laboratories. The differences are, 

 however, in detail only ; the principle throughout is the same. The details of the 

 mercury pump I have considered it best to omit. They are very complicated, and 

 the forms of pump used vary greatly in different laboratories ; the method can only 

 be learnt by actually using the instrument. 



FIG. 293. Apparatus for blood-gas 

 analysis. 



