156 LECTURES ON IMMUNITY 



recalls attention to the oft-observed fact that in gen- 

 eral positive colloids (which wander in the electric field in 

 the same manner as cations) precipitate negative colloids 

 (which wander in the same direction as anions under the 

 influence of the electric current). This reminds one some- 

 what of the specificity of agglutinins. As we shall see 

 later on, Henri accepted this idea, but later he found it dis- 

 proved by experiments on agglutinins. Biltz found, further, 

 that in the precipitation of colloids optima are found. 

 Thus, for instance, the addition of 1.62 mg. oxid of zir- 

 con gave a more abundant precipitate with 1.4 mg. gold 

 in colloidal solution than greater or less quantities; 3.25 

 mg. ZrO 2 gave no precipitate at all. In the same man- 

 ner behaved 4 mg. thorium oxid (as colloidal hydrate) 

 with a colloidal solution containing 5.5 mg. SO 2 S 3 . 



In this point also the colloids present analogies with 

 common dissolved inorganic substances. 



Analogous effects are observed as especially character- 

 istic of precipitins, and we will return to this special 

 question when we later consider them. 



Regarding the influence of salts upon agglutination, we 

 possess a thorough investigation of Bechhold. He used 

 typhoid bacilli, which were cultivated in bouillon and 

 thereafter killed with formalin and washed by repeated 

 suspension in distilled water, and followed by subsequent 

 centrifugation. These bacteria were in some experiments 

 used in their natural state. One c.c. of a suspension was 

 mixed together in a test-tube with i c.c. of the salt solu- 

 tion under investigation. The test-tube was placed for 

 24 hours in an incubator at 37 C. and its content then 

 examined. The degree of agglutination was tested by 



