144 STUDIES IN IMMUNITY. 



over, arouses certain other objections, to which we shall later refer, 

 and it appeared to us that agglutination "is due to some phenom- 

 enon of molecular physics. The slightest effects may cause chemi- 

 cal precipitates which have remained uniformly suspended in a 

 fluid to fall to the bottom of the tube. It is probable that serum 

 acts on bacteria by changing the relations of molecular attraction 

 between the bacteria and the surrounding fluid." 



It is evident that this interpretation does not explain the inti- 

 mate nature of the phenomenon any more than does Gruber's; 

 it simply compares particles of such different natures as bacteria, 

 red blood cells and chemical precipitates, each of which when sus- 

 pended in a fluid may be brought together in masses by certain 

 influences. Contrary to Gruber's conceptions, this explanation 

 implies the existence of analogies between the various forms of 

 agglutination whatever may be the substances agglutinated ; it pre- 

 supposes the predominant intervention of physical laws in the 

 phenomenon. 



We must be quite clear on this point. Does this hypothesis 

 mean that bacteria when affected by the agglutinin act simply as 

 inert particles in all phases of the phenomenon? Certainly not. 

 Agglutinins are specific; and there is, moreover, not the slightest 

 doubt that they act directly on the bacteria since these cells rapidly 

 lose their motility in the very first stages of the phenomenon. In 

 the first phase of the phenomenon the action of the agglutinin 

 evidently takes into account the particular biological nature of the 

 element that it affects : this is evident since it affects certain organ- 

 isms and not others. But the subsequent changes which bring 

 the affected organisms together may be brought about by the slight- 

 est modifications in the active substance provided they are sufficient 

 to change the relation of molecular adhesion between bacteria and 

 fluid. From that point on, according to the hypothesis, the 

 biological nature of the substances affected would no longer count. 

 The bacteria are thenceforth agglutinated according to physical 

 laws which are applicable also to certain inorganic particles, and it 

 is unnecessary to suppose the presence of an adhesive substance or 

 of sticky or viscous membranes to explain the clumping and the 

 adhesion of the micro-organisms. Gruber's hypothesis excludes 

 physical laws; our hypothesis would attribute considerable impor- 



