APPLICATIONS OF BACTERIOLOGY 255 



antityphoid serum is added, the mixture becomes turbid and a 

 precipitate forms which settles at the bottom of the tube. 

 Instead of agglutination we have precipitation (R. Kraus). 



If we inject a rabbit repeatedly with proteins of animal 

 origin, horse serum, eel's serum, defibrinated fowl's blood, its 

 serum thus immunized forms a precipitate when mixed with 

 the serum of the eel, of the horse, or of the fowl (Tchistovitch 

 and Bordet). 



The precipitating property of sera thus prepared is ascribed 

 to an antibody, precipitin. There are bacterial precipitins 

 and protein precipitins, the former being simply a species of 

 the latter : all proteins, animal or vegetable, in clear solution, are 

 precipitated by the corresponding antisera : the action is quite 

 specific. 



This reaction is nearly related to the agglutinating reaction. 

 It is scarcely ever used in the diagnosis of infections, but for 

 the differentiation of protein substances it has furnished a very 

 sensitive test which is employed in forensic medicine : nowa- 

 days serodiagnosis is applied to the detection of bloodstains 

 and to the adulteration of meat and milk. 



A man is accused of murder and in his house is found a 

 garment stained with blood : the accused (he may be, for 

 example, a butcher) declares that it is ox-blood and justice 

 demands that serodiagnosis be applied to the stains. Some of 

 the stains are therefore cut out of the cloth and extracted with 

 physiological saline and with this liquid the precipitation test 

 is applied : if the spot was due to human blood, the liquid 

 produces a precipitate with the serum of an animal previously 

 treated with human blood. 



A merchant is accused of selling for pork-sausages sausages 

 made from horseflesh. An extract is therefore made with 

 water from the suspected meat. If the sausage contained 

 horseflesh, the extract gives a precipitate with the serum of an 

 animal previously injected with horseflesh extract. The 

 method can still be applied when the meat has been smoked 

 and dried. By the same method may be detected cow's, 

 goat's milk, &c. The reaction is sensitive to i in 100,000, i.e.. 



