PRECIPITINS 



85 



(b) Identification of Blood. — An extract of the 

 blood clot is made with normal salt solution. This is 

 then filtered until absolutely clear, and diluted until 

 we have approximately a 1-1000 solution of the serum. 

 The latter point can be determined by boiling a por- 

 tion of the solution, as a 1-1000 dilution will then show 

 a slight haziness, while greater dilutions remain clear. 



One-tenth c.c. of the antihuman rabbit's serum is 

 then mixed with 1 c.c. of the suspected serum thus pre- 

 pared, and incubated for half an hour at 37° C. In 

 another tube 0.1 c.c. of normal rabbit serum is placed 

 with 1 c.c. of the suspected serum. In a third tube is 

 placed 0.1 c.c. of antihuman rabbit serum and 1 c.c. 

 of normal salt solution. The results in the case of a 

 positive reaction are shown in the following table : 



It has not been found possible to employ this test 

 for differentiation of the blood of various races of 

 human beings, such as negro from Caucasian, etc. 



The same test can be used for the blood of various 

 lower animals by preparing suitable antisera. Closely 

 related species, however, such as hens and pigeons, are 

 not differentiated by it. 



