Appendix I. On methods 309 



barometer P, the positive pressure in the tonometer 2>, the absolute temperature of 

 the bath in which the blood will be exposed to the gas 7 1 , that of the room , and the 

 required partial pressure of oxygen x, 



T 



x=Ox(P+p)x-. 



There is a slight correction which is sometimes worth making at low pressures. 

 It has reference to the fact that the air in the tonometer acquires oxygen from the 

 blood. Suppose 1'5 c.c. of blood are placed in the tonometer and the partial pressure 

 is such that half of the oxygen is given off. This will amount to 0'15 c.c., which in 

 250 c.c. will exert a pressure of upwards of half a millimetre. 



In analysing the gas after an experiment, as is frequently necessary, there are 

 some additional points to be considered. 



(1) The tonometer contains blood and this must not be allowed to foul the gas 

 burette. It is therefore advisable to connect the two by a long rubber tube of 1 mm. 

 bore. The tonometer is placed with the cork downwards ; the air is cleared from the 

 dead space of the rubber tubing, etc., by running mercury out to the open air through 

 the tonometer tap ; this is now turned and a few drops of mercury run into the 

 tonometer ; the mercury reservoir is then lowered and the gas is drawn into the 

 burette. 



(2) The positive pressure must be measured as before, but it must be borne in 

 mind that in taking the gas into the burette the pressure in the tonometer has become 

 reduced. If 8'3 c.c. are taken out of a 250 c.c. tonometer the value (P+p) will be 

 3 % below the pressure which existed in the tonometer. 



Measurement of composition of alveolar air. 



There are two methods, each of which has warm adherents, for measuring the 

 composition of alveolar air. (1) The method of Zuntz and Loewy ; (2) the method of 

 Haldane and Priestley. Before any detailed description of either of these is given 

 a few words may be said about the principles on which each rests. 



The method of Zuntz and Loewy consists in estimating the composition of the 

 inspired and expired air, and computing that of alveolar air from a number of factors 

 of which the most important is the "dead space of the lung." The dead space 

 really is the volume of the nose, throat, trachea, bronchi and bronchioles. There is no 

 profession that the alveolar air is ever actually obtained, and the correctness of the 

 method really hinges upon the accurate measurement of the dead space. 



In the method of Haldane and Priestley it is claimed that if the most sudden and 

 violent expiration possible be made, the volume of air expired is so great as to clear 

 the dead space entirely and to give a considerable volume of actual alveolar air at the 

 end of the expiration. In a sense this involves a knowledge of the size of the dead 

 space too, but in practice this may be evaded. The further one gets down the 

 passages which lead to the alveoli, the more does the composition of the air resemble 

 that of the alveolar air and the more does it depart from that of the atmospheric air. 

 If, then, as the respiration comes out, one gets to a point after which successive 

 portions of the respiration are of constant composition it may be assumed that one 

 has arrived at the alveolar air that at least is the contention ; and further it is the 

 contention of Haldane and Priestley that such a point is reached, and that about the 

 last third of the respiration consists of alveolar air, which may be collected. 



