406 RESPIRATION 



One defect of the picrocarmine tubes arose from the fact that the picro- 

 carmine is not completely stable, so that after a time its color alters. But 

 even the original standard was somewhat indefinite, depending as it 

 did on the particular percentage of haemoglobin in the sample of normal 

 blood with which it was standardized. Another defect depended on the 

 fact that the colors of the blood and picrocarmine solution are not the 

 same spectrally. In consequence of this a color match with one quality 

 of light is no longer a match with a different quality of light. Thus in 

 ordinary artificial light the reading of the instrument is quite different 

 from that in average daylight ; and in different qualities of daylight, and 

 with different observers, the match differs. The same defect exists in 

 various later forms of haemoglobinometer, where colored glass or colored 

 paper is used as a standard. By using CO haemoglobin as the standard 

 solution, and saturating the blood under examination with CO or coal 

 gas these defects are avoided. 



To avoid errors due to inequality in the diameters of the tubes, each 

 tube has first of all two marks placed on it the first at the level when 

 .2 cc. of water are introduced into a dry tube, and the second at the 

 level given by 2 cc. The distance between these two marks must corre- 

 spond exactly in the standard tube and measuring tube and this must be 

 borne in mind if either tube gets broken and has to be replaced. The 

 20 cubic millimeter pipette is also standardized by weighing on a deli- 

 cate balance. 



To make a determination, some water is first introduced into the 

 measuring tube. Twenty cmm. of blood from a prick in the finger or 

 ear are then measured into this water from the dry pipette. The blood 

 sinks, and the pipette is rinsed out with some of the water standing above 

 the blood. Some coal gas or CO is then run into the upper part of the 

 measuring tube through narrow rubber or glass tubing, and the top of 

 the tube promptly closed with the finger. With the thumb of the same 

 hand on the lower end of the tube the latter is then inverted several 

 times so as to saturate the haemoglobin completely with CO, but without 

 warming the contents of the tube. The finger can then be slid off the 

 open end of the tube without the slightest loss of liquid. More water 

 is now added by means of the dropping pipette until the tints appear 

 equal. When this point is reached the level is read off after a short inter- 

 val to allow liquid to run down. Another drop is introduced, and then 

 another, until the tints appear unequal again; and the mean of the 

 readings giving equality is taken as showing the required percentage. 

 This indicates the oxygen capacity of the haemoglobin in percentages of 

 18.5 cc. of oxygen capacity per 100 cc. of blood. 



In judging of equality in tint the tubes are held up before a window 



