n8 MICROSCOPIC METHODS. 



in B for two minutes and then to transfer to C for one and a half to 

 two minutes, and not to transfer again to B. It will also be found an 

 advantage to use a fresh supply of C for each preparation, a small 

 quantity being sufficient. The beginner will find the typhoid bacillus 

 or the B. coli communis very suitable organisms to stain by this 

 method. 



Although the results obtained by this method are sometimes 

 excellent, they vary considerably. Frequently both the organisms and 

 flagella appear of abnormal thickness. This is due to the fact that the 

 process on which the method depends is a precipitation rather than a 

 true staining. The pictures on the whole are less faithful than in the 

 other two methods. 



The Testing of Agglutinative and Sedimenting Properties 

 of Serum. 



By agglutination is meant the aggregation into clumps 

 of uniformly disposed bacteria in a fluid, by sedimentation 

 the formation of a deposit composed of such clumps when 

 the fluid is allowed to stand. Sedimentation is thus the 

 naked-eye evidence of agglutination. The blood serum 

 may acquire this clumping power towards a particular 

 organism under certain conditions ; these being chiefly met 

 with when the individual is suffering from the disease pro- 

 duced by the organism, or has recovered from it, or when 

 a certain degree of immunity has been produced artificially 

 by injections of the organism. The nature of this property 

 will be discussed later. Here we shall only give the 

 technique by which the presence or absence of the 

 property may be tested. There are two chief methods, a 

 microscopic and a naked eye, corresponding to the effects 

 mentioned above. In both, the essential process is the 

 bringing of the diluted serum into contact with the bacteria 

 uniformly disposed in a fluid. In the former this is done 

 on a glass slide, and the result is watched under the 

 microscope ; the occurrence of the phenomenon is shown 

 by the aggregation of the bacteria into clumps, and if the 

 organism is motile this change is preceded or accompanied 

 by more or less complete loss of motility. In the latter 

 method the mixture is placed in an upright thin glass tube ; 



