316 Quantitative Precipitin Tests 



By using the fingers of both hands in rotation eight tubes can be shaken 

 in succession, after which the hands must be cleansed before proceeding 

 to the next set of tubes. This is only mentioned because it saves time. 

 With a little practice it will be found easy to bring about complete 

 mixture without the formation of troublesome air-bubbles, and if the 

 fluid is shaken down to the bottom of the tube before removing the 

 finger the loss of fluid entailed by contact with the finger is practically 

 negligible, the finger-tip remaining apparently dry after some practice. 

 The mixture is now left standing at a desired temperature, the rate of 

 precipitum formation, etc. being noted. 



After 24 hours have elapsed the precipitum has become deposited 

 upon the bottom of the tube. Should any adhere to the walls of the 

 vessel it may be dislodged by gently tapping or rotating the tube upon 

 its vertical axis. The particles thus displaced soon sink to the bottom 

 of the tube, provided it does not contain too much serum, which through 

 its viscidity impedes their gravitation. The clear supernatant fluid is 

 now pipetted off by means of a conveniently bent bulb capillary pipette, 

 the tip of which is allowed to slide down along the inner wall of the test- 

 tube, the precipitum not being disturbed, the clear fluid being aspirated 

 into the bulb as the tip of the pipette slides down a little beneath the 

 receding surface of the column of fluid in the tube. The bulk of pre- 

 cipitum and clear fluid left is so small that it can now be readily drawn 

 up into the capillaries. 



The capillaries used for measuring the precipitum volumetrically are 

 made of fairly thick glass, are about 12 cm. long, and have a lumen of 

 about 1 mm. The ends should be cut off square by means of a diamond, 

 this being essential, as it is thus possible to practically remove all the 

 fluid and precipitum in the bottom of the 'small test-tube. The end of 

 the capillary is placed against the bottom of the tube, and the deposit 

 and fluid are well mixed by being drawn backwards and forwards into the 

 capillary. It is imperative that bubbles should be avoided. The fluid 

 is now drawn up a little way into the capillary, and the dry end of this 

 is drawn out and sealed in a very small flame. The sealed end should 

 be blunt to avoid possibility of breakage ; the other end of the tube, 

 which can now be cut down to a convenient length, is left open. After 

 a number of capillaries have been thus prepared they are placed verti- 

 cally in a rack in a cool place. After 24 hours the height of the column 

 of precipitum is marked upon the capillary with a glass pencil. After 

 48 to 72 hours it will be seen that the column of precipitum remains 

 constant. The amount of precipitum may now be measured. 



