144 IMMUNE SERA 



formed ; they are then slightly tilted, and as soon as 

 serum enough has been expressed, this is pipetted 

 off into sterile test containers which are stored in a 

 cool place. It is well not to add any preservative 

 to the serum, as such an addition may occasionally 

 lead to pseudo reactions. 



The Test. In carrying out the test the sus- 

 pected clot is mixed with a small quantity of normal 

 salt solution and then filtered. Whether or not the 

 blood specimen has gone into solution can best be 

 judged by the foam test. Air is blown gently through 

 the pipette which is used for transferring the solu- 

 tion into the test-tubes. Solutions of blood or 

 serum of i : 1000 and over, still foam well. The color 

 of the fluid is not so reliable an index of solution. 

 To some of this solution in a test-tube, about double 

 the amount of the specific serum (derived as above) 

 is added. As a control test, we place a little blood 

 of another species, e.g., of an ox, in a second test- 

 tube together with some of the specific serum and a 

 little normal salt solution. In a third tube we 

 place some of the suspected blood solution, and in a 

 fourth some of the specific serum mixed with the 

 normal salt solution. All four tubes are placed in 

 the incubator at 3 7 C. for one hour, or are left at 

 room temperature for several hours. If the sus- 

 pected clot was one of human blood, the first tube 

 will show distinct evidence of precipitation, while 

 all the control tubes will have remained clear. It 



