THE AGGLUTININS 217 



volume of emulsion of B. coli, agglutination occurred when between 

 118 centigrammes and 4 centigrammes of the acid was present, 

 and when between i-i milligrammes and 0*001 milligramme, but 

 not with intermediate amounts. 



Similar phenomena (i.e., the presence of zones of inhibition) can 

 be seen in many of the actions of coagulants on colloid emulsions 

 or solutions, such, for instance, as the precipitation of gum mastic 

 from water by ferric chloride or trisulphide of arsenic. This 

 case also, as Dreyer points out, shows a marked analogy with 

 the clumping of coli bacilli by phosphoric acid, since in each case 

 the zone of inhibition becomes smaller when the agglutinating sub- 

 stance (bacilli or particles of gum) become more numerous. This 

 analogy between the agglutination of bacteria and the flocculation 

 of colloids has been investigated by Bechhold, Neisser and 

 Friedemann, and Biltz, and is a subject of the utmost interest, 

 and one which bids fair to revolutionize our views on the inter- 

 relations of the antigens and antibodies. At present our knowledge 

 of the subject is hardly sufficiently advanced to justify an account 

 of the experiments and theories on which these views are based, 

 and the original papers must be consulted for further information. 



To revert again to the question of the specificity of the reaction : 

 The subject is a complex one, and we have already seen that the 

 phenomenon of the "group reaction " leads us to the supposition 

 that bacteria of different species must possess molecules of the 

 same nature. Further study shows that there are differences in 

 this respect between bacteria of the same species, which are in- 

 distinguishable the one from the other by ordinary morphological 

 and chemical tests, but which have had different origins. Thus it 

 is found that if the serum of an animal which is strongly immu- 

 nized to a given culture of V. cholera: be tested against cultures of 

 various stocks, that which was used for the injections will be 

 clumped most powerfully, the others to variable degrees. These 

 apparent anomalies, though inconvenient in practice, tend to make 

 us regard the reaction as more specific rather than as less 

 sharply specific, that is, as regards a certain sort of chemical 

 substance which is formed in greater degree by certain races of a 

 given species than by others. 



This modified specificity, sharp as regards chemical substances, 

 but not as regards bacterial species, is well illustrated in 

 Castellani's absorption reaction. If an agglutinating serum which , 

 e.g., clumps the typhoid bacillus powerfully, and a colon less 



