INTERPRETATION' OF THE PRECIPITIN' REACTION. 321 



cipitate marks a further advance in accuracy. The remark- 

 able nature of the precipitate is thus revealed. For, if our 

 interpretation be correct, the precipitate is formed mainly 

 of those constituents of the antiserum that give the anti- 

 serum its specific precipitable character. It is probable 

 that the specific a ntisuhstauee (precipitin) brings down with 

 it from the antiserum non-specific molecules (e.g. comple- 

 ment) which also go to form the precipitate. It is probable 

 also that the antigen enters to a relatively slight extent 

 into the composition of the precipitate. But all our work 

 tends to show that the main mass of the precipitate is 

 composed of antisubstance (precipitin) which can be weighed. 

 A ponderable antisubstance has hitherto been unknown, 

 but in the precipitate of a precipitin reaction we have a 

 remarkably near approximation to such a substance. 



Phenomena of Precipitation : («) relation of antiserum 

 to precipitate.— The original interpretation of the precipitin 



"precipitin" which had the power of throwing the antigen 

 (homologous protein) out of solution. " Tres generalement, 

 en effet, le serum d'un animal d'espece A, injectede serum 

 d'espece B, precipite ce serum B " (2). It was admitted 

 that some portion of the precipitate might be derived from 

 the antiserum (3, 4, 5, 6). It was also said that the pre- 

 cipitin combined in equivalent quantities with the antigen 

 to form the precipitate (7). Nevertheless the fundamental 

 assumption was made that the main source of the precipi- 

 tate was the antigen (homologous protein) which was 

 therefore known as the "prefipitablc -aihstance." It was 

 suggested that the precipitin reaction was analogous to 

 bacterial agglutination, and that, in some similar manner, 

 precipitins agglutinated and precipitated protein molecules 

 from solutions of the antigen (8, 9). 



At an early stage of our work on precipitins we found 

 that this interpetation of the reaction was not borne out 



