230 PRECIPITOID 



precipitate i.e., the precipitoids were at this point just saturated ; 

 but in a mixture of 20 parts of antiserum with i part of normal 

 serum, it appears that, after all the molecules of precipitoid were 

 saturated, there was still enough precipitable substance present to 

 combine with some of the precipitin, and thus to form a precipitate. 

 In the mixture of 8 and i the precipitoid was all saturated, and a 

 maximum amount of precipitable substance left over to combine 

 with precipitin. 



This theory of the " specific inhibition " of the action of this 

 precipitin was the first explanation to be brought forward, and 

 appears to afford a fairly satisfactory explanation of the facts 

 observed. It is, however, quite probable that future physico- 

 chemical research may show it to be erroneous, and that these 

 zones of inhibition are in reality similar to those observed in the 

 non-specific precipitation of colloids, of which we have already 

 spoken. To this subject we shall return. Be the explanation 

 what it may, the phenomena are of considerable practical impor- 

 tance in the application of the precipitating sera to the diagnosis 

 of the nature of an unknown serum or other solution of proteid. 

 The mere fact of not obtaining a precipitate when a solution of 

 unknown strength (e.g., of a dried blood-stain) is added to an 

 anti-serum is not necessarily of any importance, and to obtain 

 accurate results it is necessary to perform a series of quantitative 

 experiments. 



The experiments of Eisenberg, Michaelis, Fleischmann, and 

 others tend to show that the combination between the two obeys 

 the laws governing the combination of weak acids and bases, 

 which we have already discussed, and that when the two sera are 

 added together both free precipitin and precipitable substance may 

 be present at the same time. This, if true, would not explain 

 relations between the precipitin and precipitable substance similar 

 to those given above ; if the law of the mass reaction applied, 

 an excess of antiserum would tend to give the maximum 

 possible quantity of precipitum, though there would always be 

 some unaltered precipitable substance present in an unaltered 

 state. It is, however, doubtful whether the substances do actually 

 interact as weak acids and bases. Von Dungern, experimenting 

 with antisera obtained to the proteids of cold-blooded animals, 

 found that the reaction was rigorously quantitative, but that a 

 complication was introduced by the presence of two varieties of 

 precipitin, special and partial. The special precipitins are sup- 



