PRECIPITINS. 241 



titious precipitins in such small amounts that 

 they do not interfere with the chief precipitin. 



That feature of the precipitation reaction which Forensic 

 has the most practical bearing has to do with its Prectpitii 

 medicolegal use in the detection of human blood. 

 For this purpose it has supplanted the specific 

 hemolytic serums, which are to be referred to later. 

 In the course of investigations it was found that 

 even the smallest dried blood stain, although 

 months old, would cause the formation of a sedi- 

 ment when mixed with its homologous precipitat- 

 ing serum. It remained for certain important de- 

 tails to be worked out in order to render the test 

 sufficiently reliable for forensic work. The spe- 

 cificity of the reaction appeared to be threatened 

 somewhat when it was learned that the serum of 

 monkeys undergoes precipitation when treated by 

 an immune serum which is specific for human 

 serum. This is, again, group precipitation. Ad- 

 ventitious precipitation is, in fact, so widespread 

 that some have felt justified in speaking of a mam- 

 malian serum reaction. Hence, in order to insure 

 specificity, it has become necessary to use precise 

 quantitative methods in differentiating bloods or 

 serums by this method. The immune serum which 

 is used in the test must be diluted to some extent 

 in order to eliminate accidental precipitins; but 

 even a more important precaution is the volumet- 

 ric measurement of the precipitate which is 

 formed. The technic of Schur may be cited. Test 

 tubes are so made that the lowermost portion con- 

 sists of a graduated capillary tube. One c.c. of the 

 fluid to be tested is placed in one of these tubes, to 

 which is then added 0.2 c.c. of the precipitating 

 serum. The mixture is kept at body temperature 



