244 INFECTION AND IMMUNITY. 



the positive or the negative pole), and, moreover, 

 carry electric charges themselves. 



Precipitation The features of colloids which bring them into 

 Electrolyte^ relation with the subject in hand are their coagu- 

 lable nature in certain instances and the fact that 

 their particles may be agglutinated or precipi- 

 tated by the addition of minute amounts of salts 

 (electrolytes). In this connection one naturally 

 recurs to the observation of Bordet, which was 

 mentioned in the preceding chapter, concerning 

 the inagglutinability of micro-organisms so long 

 as salt is withheld from the solution. This anal- 

 ogy would suggest that the bacteria after their 

 union with agglutinin may conduct themselves as 

 colloidal particles. In the precipitation of colloids 

 by salts it has been suggested that the salts so 

 alter the electric condition of the colloidal parti- 

 cles that their surface tension is decreased, and as 

 a result of this change neighboring particles 

 coalesce in such quantities as to produce a visible 

 sediment. 



Neisser and Friedberger have studied certain 

 colloids, having in mind the similarity of their be- 

 havior to serum reactions. They found, for exam- 

 ple, that two of our common dyes which are col- 

 loids and bear opposite charges of electricity (eosin 

 and Bismarck brown), give rise to a precipitate 

 when the two are mixed. Furthermore, the spe- 

 cific inhibition which may be obtained in the reac- 

 tion with serum precipitins (see above) could also 

 be realized with the eosin and Bismarck brown. 



The agglutination of bacteria and of red blood 

 cells may also be accomplished with colloids. 

 Landsteiner agglutinated erythrocytes with col- 

 loidal silicic acid. 



