140 METHODS FOR THE ANALYSIS OF BLOOD 



the bag at each inspiration. The observer should indicate when 

 to breathe in and out. Breathing more frequently will not greatly 

 alter the results. At the end of twenty seconds, the tube is clamped 

 off and the air analyzed. The analysis should be carried out 

 within three minutes' time, as carbon dioxide rapidly escapes 

 through rubber. 



Analysis of Sample. In analyzing a sample of air, about 2 

 or 3 c.c. of the standard bicarbonate solution are poured into a 

 clean test-tube of the same diameter as the tubes containing 

 standard phosphate solutions, but from 100 to 150 mm. long. 

 Air from the bag is then blown through the solution by means 

 of a glass tube drawn out to a fine capillary point, until the solu- 

 tion is saturated, as shown by the fact that no further color change 

 occurs. (If the operator first blows his own breath through the 

 solution so as to bring it into approximate equilibrium with alveolar 

 air, saturation may be accomplished with as little as 100 c.c. of 

 air from the bag, blown through during thirty seconds. The 

 same bicarbonate solutions may be used for repeated determi- 

 nations.) The tube is stoppered and the color immediately com- 

 pared with that in the standard tubes. By interpolation, one can 

 readily read to millimeters. Color changes are not quite so sharp 

 above 35 mm. as at the lower end of the scale, but here changes 

 are of less significance. In making the color comparisons the 

 solution being compared is placed between the two standards 

 which it most nearly matches. When there is doubt as to whether 

 the color of the solution is higher or lower than one of the standards, 

 changing the order in which the tubes are placed in the comparison 

 box will generally make the relationship clear. 



The standard solutions described are so prepared as to give 

 correct results when the determination is carried out at a tem- 

 perature of from 20 to 25 C. (from 68 to 77 F.). When the 

 room temperature is considerably higher or lower than these 

 points it is advisable to immerse the tubes in water at approxi- 

 mately 25 C. during the blowing. They may be removed from 

 the water for the color comparison, however, provided this is 

 quickly made. The differences due to ranges of temperature occur- 

 ring under ordinary circumstances are practically negligible. 

 (No correction for barometric pressure is required as from the 

 nature of the determination, barometric fluctuations are self- 

 corrective. Variations in the temperature of the subject are never 



