PRECIPITATION TEST 171 



cotton soaked in liq. ferri sesquichloridi (ferric chloride) or by 

 placing a small hemostat for % to i hr. on the incision. The 

 serum which has separated from the clot is centrifugalized and 

 the titer is determined as follows: 



Preliminary Titration. Into each of a series of six test-tubes 

 place 2.0 cc. of the following dilutions of serum (horse or deer) 

 antigen, mixed with 0.85 per cent, saline 1:100, 1:500, I:IOOQ, 

 i : 5000, i : 10,000 and i : 20,000. To each cubic centimeters of the 

 dilution o.i cc. of antiserum is added. The solution of 1:1000 

 should become turbid instantaneously or within i to 2 min., the 

 other dilutions in from 3 to 5 min. The serum should have a 

 titer of 1:20,000; that means the serum should cause a turbidity 

 in a dilution (of horse serum or extract of meat) of i : 20,000 in less 

 than 5 min. The antiserum is either introduced by allowing 

 it to run down the side of the tube (no shaking is permissible), 

 or it is stratified on the diluted horse serum. In the first case the 

 turbidity appears from the bottom, in the second case in form of a 

 grayish ring; both reactions are positive. The coloration is 

 best seen against a dark background. The pipettes and test- 

 tubes must be perfectly clean and sterile. The equipment de- 

 signed by Uhlenhuth is very satisfactory. The test-tubes are 

 long and narrow, 10 cm. by 0.8 cm., and are suspended in beveled 

 holes of the test-tube rack. Pipettes of i cc. capacity graduated 

 into Koo cc., and 5 and 10 cc. pipettes graduated into Jf<) cc - 

 will be found satisfactory. 



Preservation of Serum. In case the titer of the serum is 

 satisfactory, the rabbit is bled to death (aseptically) from the 

 carotids. For full details on technique, consult the text-books on 

 Immunity. The centrifuged serum should be perfectly clear 

 and sterile and should not be opalescent. Kept cool and in the 

 dark (ice chest) it will remain potent for months, even years. 

 To avoid opalescence the animal should be bled only after a period 

 of fasting. On account of autoprecipitation, it will lose some of 

 its potency. The precipitate formed can be removed by cen- 



