180 METHODS FOR THE ANALYSIS OF BLOOD 



for at least half an hour. Four beads about 3 mm. in diameter 

 have been found the most satisfactory number for each tube in 

 the determination. With use they become etched by the strong 

 acid and are then more effective in promoting even boiling than 

 when new. 



If the heating of the blood with the acid mixture is hastened 

 too much, the nitric acid may be driven off before oxidation is 

 complete as shown by a darkening of the solution. If this 

 occurs one or two drops of nitric acid are added, and the heating 

 is continued. 



The use of the cane sugar is necessary because of the formation 

 of a varying amount of some compound of phosphoric acid (prob- 

 ably a nitric acid compound) which does not precipitate with the 

 reagent and which is destroyed by organic material added. In 

 the determination of lipoid phosphoric acid the sugar appears 

 to be less necessary, but in determinations of total phosphates 

 low values are invariably obtained unless it is used, and it is safest 

 therefore to use it as a routine procedure in all determinations. 



During the boiling of a H^SO.* water mixture, first the excess 

 of water is boiled off, then the water vapor is succeeded by a dense 

 white cloud of mixed water and sulfuric acid vapor, and finally 

 by the much thinner vapor of the sulfuric acid alone. In this 

 final stage a clear zone appears between the liquid and the vapors 

 above. The appearance of the clear zone indicates that the heat 

 is strong enough. 



Accurate nephelometric measurements are possible only when 

 the size of the particles of the suspension is approximately the 

 same in both standard and test solutions. This similarity is secured 

 by adjustments of conditions as regards salt content, temperature, 

 etc., which are made as nearly as possible the same in both 

 solutions. 



Three minutes is the minimum time for the precipitation to 

 come to the point where it may be measured. The length of time 

 that the solutions may stand and still give accurate readings 

 has not been determined, but in solutions no more than 25 per 

 cent apart no significant change has been found in twenty minutes. 



The jackets of the nephelometer should fit snugly at the top 

 so that the light is cut off sharply at that point and there should 

 be the minimum of play in either tubes or jackets, otherwise con- 

 sistent readings are not obtained. It is rarely possible to get 



