202 COLLOIDS IN BIOLOGY AND MEDICINE 



what more obvious. Inasmuch as the two dissolved colloids yield 

 a precipitate when mixed in suitable proportion, we can form a 

 judgment concerning the proportionate quantities that combine. 

 However, this has regard for the composition after precipitation. Ac- 

 cording to E. VON DUNGERN* the precipitate binds much more precip- 

 itin than is required to cause complete precipitation. It is still a 

 question whether this combination existed in the solution. 



C. Precipitation of Dissolved Colloids and Organized 

 Suspensions, 



Serum containing precipitin, for instance, goat-rabbit serum, 

 gives a precipitate with its antigen (goat serum). The serum is 

 precipitated by the precipitin just as it would be by an inorganic 

 hydrosol or an acid protein (histone), U. FRIEDEMANN and H. FRIEDEN- 

 THAL* (see p. 157). The precipitation occurs best in the presence of 

 an optimum mass proportion between precipitin and precipita,ble 

 substance; excess of precipi table substance interferes with the pre- 

 cipitation. A precipitation, according to M. NEISSER,* occurs also 

 in salt-free solution (which contains no globulin) but the precipi- 

 tation zone differs from that in a solution containing salt. 



Though the mutual precipitation of two amphoteric colloids de- 

 pends on the hydrogen ion concentration (see p. 147), specific precip- 

 itations (this applies to precipitins and agglutinins) are largely in- 

 dependent of it (L. MICHAELIS and DAVIDSOHN). The electric charge 

 of the components plays a very subordinate part in these precipitations. 



The plant toxins, ricin (from the seeds of varieties of castor bean) 

 and abrin (from jequirity seeds) have a similar precipitating effect 

 upon albumin. 



The conditions in the case of organized bacterial albumin are 

 similar to (but not identical with) those of serum albumin. If an 

 animal (e.g., a rabbit) is injected with bacteria (for instance, typhoid 

 bacilli) there develops in its blood an agglutinin which causes typhoid 

 bacilli in a test tube to precipitate. 1 Agglutinin forms a compound, 

 with the (actual) albuminous capsule of the bacteria, so that these 

 behave as though they were changed from a hydrophile to a 

 hydrophobe suspension. Precipitation occurs only in water con- 

 taining salt. 2 Though a suspension of bacteria is unchanged by 



1 This phenomenon was first observed by GRTJBER and DURHAM. WIDAL was 

 the first to use it for diagnosis, and since, as the Gruber-Widal Reaction, it is em- 

 ployed in the diagnosis of typhoid, paratyphoid, dysentery, etc. 



2 According to U. FRIEDEMANN it is possible to obtain agglutination in a salt- 

 free solution, though this has nothing in common with specific agglutination. 

 The resemblance to the precipitins is, in this respect, only a superficial one. 



