

 AGGLUTINATION AND PRECIPITATION 155 



applications have been made of precipitation in the differentiation 

 of proteins. These are the recognition of blood-stains, the differ- 

 entiation of meats from different species of animals, particularly 

 horse-flesh from beef, and the diagnosis of many other protein- 

 containing substances, including bacteria and their products. 



Recognition of Blood-stains. It is sometimes necessary in 

 murder trials to determine with certainty the origin of a blood-stain. 

 The fact that the stain has been produced by blood may be easily 

 demonstrated by the chemist, but he has no ready means of telling 

 with certainty whether the blood is of animal or human origin. 

 Uhlenhuth was the first to call attention to the value of this test 

 in legal medicine. The precipitation test, when properly carried 

 out, enables the determination to be made with a high degree of 

 certainty. An antiserum specific for human blood must be se- 

 cured first by injections of human serum into a rabbit, at intervals 

 of a few days, for a period of several weeks. A bit of the material 

 with the blood-stain is placed in a watch-glass and 5 c.c. of sterile 

 physiological salt solution is added. This is allowed to stand until 

 some of the blood proteins have been dissolved. This may be 

 shown by blowing into the liquid through a capillary tube, when a 

 fairly permanent foam will be produced. If any dirt or sediment 

 appears in the solution, it is removed by filtration. An effort 

 is made to secure a dilution of the serum of about 1 : 1000. The 

 diluted serum is placed in a series of test-tubes, and the specific 

 antiserum is added. If the blood-stain is from human blood, the 

 precipitate will make itself apparent as a clouding in the course of 

 a few minutes. The reliability of the test has been recognized 

 by the German courts, and the results have been accepted as 

 evidence. By varying the procedure, the same method may be 

 used in differentiating animal bloods. 



Differentiation of Meats. Meat inspection, particularly in 

 certain European countries, includes the differentiation of meats. 

 In certain localities large quantities of horse-flesh are used for food, 

 but the law forbids that horse-flesh be sold as beef. There are cer- 

 tain chemical differences between the two, differences in the com- 

 position of the fat and possibly in the abundance of glycogen, but 

 these differences require careful chemical analysis and examination 

 for their recognition. The precipitation test furnishes an easier 



