BLOOD AND THE IDENTIFICATION, ETC. 619 



to another ; groups of three or four are thus formed which 

 then adhere to other groups, until, if the reaction be com- 

 plete, all the micro-organisms are collected into large 

 clumps, with total loss of movement. Should the serum 

 be very potent and not sufficiently diluted, agglomeration 

 and loss of movement may occur instantly, so that by the 

 time the slide is brought under the microscope the reaction 

 is already completed. On the other hand, with a weak 

 or much diluted serum, the phenomena may remain per- 

 manently incomplete, so that several isolated micro-organ- 

 isms are still seen, and even some of those in the clumps 

 may retain their motility. So delicate, however, is the re- 

 action that serum from a very highly immunised animal will 

 produce an effect on its corresponding bacillus even when 

 diluted 500,000 times. The action is to a considerable 

 degree specific (or as Durham prefers to terni it, special), 

 for a "cholera" serum will not act on typhoid bacilli, and 

 "typhoid" serum will not act on coli bacilli. On the other 

 hand, " cholera " serum will affect to some extent certain 

 vibrios which are not cholera, and "typhoid" serum will 

 agglomerate B. enteriditis. But there is generally some 

 difference, and any given serum acts best, viz., in greater 

 dilution, on its own kind of bacillus, so that it can be used 

 for its identification, without causino- confusion. 



The extent to which any serum can be diluted and yet 

 show an effect depends in part on the virulence of the cul- 

 ture employed ; the more attenuated the culture, the weaker 

 the serum solution necessary to produce an effect. It is to 

 this grouping power that Durham gave the name " clumping 

 action " of the seurm, and the hypothetical body producing 

 it was termed by Gruber "agglutinin". 



It is obvious that the reaction can be applied to the 

 identification of various kinds of bacilli, if we possess 

 specimens of the corresponding "immune" sera; and 

 perhaps, as will be indicated later, it may be of use in the 

 discovery of new forms. In spite, therefore, of the sources 

 of error just mentioned, a "typhoid" or "cholera" serum 

 can prove of great use in determining whether doubtful 

 organisms belong to these species. It is also clear that 



