RESPIRATION 419 



When looked at against the sky, the solution will now have a deep 

 purplish-pink tint, as compared with the brownish yellow of the normal 

 blood solution. The carmine is now added from the burette to the normal 

 blood solution until its tint is about equal in quality to that of the satu- 

 rated blood solution. It will then probably be found that the depth of 

 tint is too great in the tube containing the carmine. Water is then added 

 from the other burette until the depth of tint is equal, and if necessary 

 more carmine, until complete equality of both tint and depth of color is 

 obtained. In judging of this, the test tubes should be held up against the 

 sky, and it is absolutely necessary to change them repeatedly from side 

 to side ; otherwise gross error is certain. It will nearly always be found 

 that the right-hand tube appears a little yellower or pinker than the left- 

 hand one ; and a little deeper or less deep in color. This difference is in 

 reality a great help to accuracy. A point is first reached when the tubes 

 appear equal in tint or depth when held in one position, but unequal in 

 the other, and the end point, when the difference is the same on one side, 

 whichever tube is on that side, can be estimated with great delicacy. 

 The additions of carmine (or water) are continued until this point is 

 passed ; and if two successive additions both show equality, the mean of 

 the two readings is taken as correct. 



To the carmine solution in the measuring cylinder a proportion of 

 water is now added equal to what had to be added from the water 

 burette to the carmine required to reach the end point of the titration. 

 The carmine solution is then ready for use. It will probably be found 

 that about 6 cc. of carmine are needed to reach the end point. The 

 amount required varies, however, according to the condition of the 

 strong carmine solution and the quality of the daylight. The carmine 

 solution is not stable, and it gradually becomes less deep in color, and 

 redder in tint than when first prepared. Hence the quantity of carmine 

 solution needed increases from month to month, and the extent to which 

 it has to be diluted for use diminishes. If the dilute solution is left for a 

 day or two exposed to light it becomes very markedly redder and more 

 dilute. 



The titration of a blood sample is carried out as follows. One or two 

 drops of blood are needed, and are at once diluted with water. Half of 

 the dilute solution is poured into one of the two test tubes (always the 

 same one as that used for the saturated blood in standardizing the 

 carmine), and 5 cc. of the normal blood solution are measured with a 

 pipette into the other. Water is then allowed to drip from a tap into the 

 solution of the blood under examination until its depth of tint is about 

 equal to that of the normal solution. Carmine solution is now added to 

 the normal blood solution from the burette until the tints are equal, 



