TECHNIC OF PRECIPITIN REACTIONS 329 



necessary that whole blood be injected, as the serum alone will suffice. 

 It is better to inject a number of rabbits with each serum and to give all 

 injections intravenously. From the third injection on, preliminary 

 titrations are made according to the technic to be described later, as 

 many animals succumb after a large number of injections have been 

 given them. 



The serum must be absolutely clear. Animals should be bled after 

 a period of fasting, as the opalescence of the serum following feeding 

 cannot be removed by filtration and will interfere with the reaction. 

 Precipitin immune serum should be collected with a scrupulous aseptic 

 technic, and stored in ampules holding 1 c.c. Although it is best not to 

 add a preservative, the addition of 0.1 c.c. of a 1 per cent, solution of 



FIG. 92. TEST-TUBE RACK FOR PRECIPITIN AND AGGLUTINATION REACTIONS. 

 The strips of black material in the rear of the tubes facilitate reading the reactions. 



phenol to each cubic centimeter of serum does not render the fluid cloudy 

 and aids greatly in its preservation. 



If, after long standing, a precipitate has become deposited in an anti- 

 serum, this should not be shaken up, but the ampule should be carefully 

 opened and the clear supernatant serum drawn off with a capillary pipet. 

 A serum that has become cloudy may be cleared partially or entirely by 

 filtering it through a small candle filter, although even an infected and 

 offensive serum will give the reaction (Fig. 91). 



Apparatus. Long and narrow test-tubes, 10 cm. by 0.8 cm., are 

 used. These must be absolutely clean, and preferably sterile. 



The test-tube rack devised by Uhlenhuth, in which the tubes hang 

 suspended in beveled holes, is quite satisfactory. Where a test is carried 

 out with many controls, a rack similar to the one shown in the illustra- 

 tion (Fig. 92) is quite serviceable. A strip of black material placed be- 



