328 



PRECIPITINS 



tion that must be added to a portion of the extract to reduce it to a 

 dilution of 1 : 1000 should be determined. The extract should be almost 

 colorless by transmitted light, and must be crystal clear; this may be ac- 

 complished by filtering it through a clean sterile Berkefeld filter. It is 

 highly desirable that the extract be sterile, as the reaction may require 



FIG. 91. AN UHLENHUTH FILTER. 



Serum is poured into the glass cylinder surrounding the "candle"; a vacuum is 

 produced by the water-pump; the nitrate is collected in the small bottle attached by 

 rubber tubing to the graduated cylinder at the lower portion of the apparatus. This 

 filter is especially adapted for filtering small amounts of serum or other fluid. 



several hours, and cloudiness due to bacterial growth may interfere with 

 the result. 



The Immune Serum. A highly potent, sterile, and absolutely crys- 

 tal-clear serum immune against the protein to be recognized must be 

 prepared. The method of preparation is given in the chapter on Active 

 Immunization of Animals. For the recognition of blood-stains it is not 



