THE LIVING ORGANISM 213 



to demonstrate by any ordinary chemical 

 reaction known to us, but easily demonstrable 

 by this bio-chemical test. This inimical 

 substance has been suddenly presented to the 

 enteric organisms cultivated artificially out- 

 side, and this agglutination is the result. 

 The agglutination is probably due to altera- 

 tions in what is called surface tension at the 

 interface between the organism and the fluid 

 in which it is immersed. The reaction is 

 so delicate and specific that it will occur even 

 when the enteric serum is diluted two-hundred- 

 fold in a typical case. First discovered for 

 the enteric organism, this property of agglu- 

 tination has since been extended to many 

 others, and is now used as a routine method 

 of settling the identity of suspected organisms, 

 as well as for diagnosing the nature of indivi- 

 dual cases of disease. Nearly allied organisms 

 cause mutual agglutination, each for the 

 other, in fluids in which cultivations have 

 been previously made, and the degree of 

 dilution at which agglutination occurs indi- 

 cates the closeness of relationship in a 

 group, or complete identity. Organisms more 

 remote have no effect upon one another. 

 It is thus seen, even at the level of bacteria, 

 how complex and finely balanced the structure 

 of the colloids has become. It will presently 



