THE PHENOMENON OF PRECIPITATION 269 



of complement fixation, exert definite complement-binding power. 

 Moreover, it has not been experimentally shown that colloidal sub- 

 stances react in accordance with the laws of mass action as observed 

 for simpler chemical substances. 



As regards the opinion of von Dungern, this seemed incom- 

 patible with another occurrence, observed by many writers, namely, 

 that such sera, although clear at first, eventually, after prolonged 

 standing, do actually precipitate spontaneously; that is, the union 

 of the precipitin and the precipitinogen does actually take place, but 

 goes on with extreme slowness. 



Now a notable and strange feature of this phenomenon is the 

 fact that two such sera, both containing antigen and precipitin, but 

 neither of them precipitating by itself, will precipitate each other 

 when mixed. For this reason Uhlenhuth has advised against the 

 use of mixtures of precipitin sera for forensic tests. For it is not 

 unusual that precipitin sera, even when produced by the slow method, 

 may contain traces of antigen, and this may lead to precipitate 

 formation if such a serum is mixed with another homologous pre- 

 cipitin and thereby simulate a positive forensic test. 



In seeking analogy for this serum phenomenon with the various 

 colloidal suspensions, the problem consisted in protecting two 

 mutually precipitating colloids by a third, and this in such propor- 

 tions that the mixing of two such protected suspensions, each con- 

 taining all three of the elements, would be followed by precipitation. 

 This was obtained by the use of gum arabic, gelatin, and arsenic tri- 

 sulphid. Thin emulsions of gelatin will precipitate arsenic tri- 

 sulphid suspensions. Small amounts of gum arabic will act as a 

 protective agent, preventing the precipitations. 



The amount of the protecting substance necessary to prevent 

 precipitation in any one mixture varies apparently with every 

 change in the relative proportions of the two. Thus a considerable 

 number of mixtures of the three can be made which will remain 

 stable for days, the actual and relative quantities of the three 

 ingredients differing in each of the mixtures. When two such mix* 

 tures are poured together, in many cases precipitation will result, 

 varying in speed and completeness, according to the particular quan- 

 titative relationship arrived at in the mixture. 



An example of such an experiment follows : 



Two solutions of colloidal arsenic sulphid were prepared, one containing 

 1 gm. per liter, the other containing 5 gm. per liter. With Kahlbaum's "Gold- 

 ruck" gelatin a solution containing 1 gm. per liter was prepared. A solution of 

 gum arabic was prepared which contained 10 gm. per liter, this being made 

 stronger than the gelatin solution to avoid too great dilution in the final mixtures. 

 The gelatin solution was prepared twenty-four hours before being used, as 

 freshly prepared gelatin has but slight precipitating power for arsenic sulphid, 

 this power appearing to increase greatly with the ageing of the solution. 



