232 INFECTION AND IMMUNITY. 



ulation occurs in the envelope, rendering coales- 

 cence with the envelopes of other individuals pos- 

 sible. The theory of Paltauf that the agglutinable 

 substance finds its way to the surface of the bac- 

 terium and is precipitated by its union with ag- 

 glutinin is somewhat similar. The shell of the co- 

 agulated substance accounts for the sticky charac- 

 ter which the envelope acquires, according to the 

 theory of Gruber. Paltauf cites observations 

 which tend to show that some substance actually 

 is extruded from the micro-organisms during ag- 

 glutination, and that in properly stained speci- 

 mens it can be seen as a precipitate surrounding 

 and between adjacent organisms. 



The multiplicity of theories leads one to suspect 

 that the true nature of the process remains obscure. 

 The physical nature of the reaction is strongly 

 supported by the facts that bacteria may also be 

 agglutinated and precipitated by well-known chem- 

 ical substances, such as hydrochloric acid, and by 

 various organic and inorganic colloids (colloidal 

 solutions of calcium chlorid (CaCl 2 ), zinc sulphate 

 (ZuSo 4 ), ferric hydroxid (Fe(OH) 3 ), aluminum 

 hydroxid (Al(OH),), ferric chlorid (FeCl 3 ) and 

 aluminum chlorid (A1C1 3 ). Some of these sub- 

 stances behave like the agglutinating serums in 

 the possession of the so-called prozone; i. e., they 

 may fail to agglutinate in more concentrated solu- 

 tions, whereas after dilution, their agglutinating 

 power becomes manifest (hydrochloric acid and the 

 staphylococcus, according to Buxton and Eahe). 

 Still further indirect evidence of this nature of 

 the reaction is found in the observation, made first 

 by Neisser and Friedberger, that two colloids which 



