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On the Main Source of '" Precipitable " Substance and on the Role 

 of the Homologous Proteid in Precipitin Reactions. 



By D. A. Welsh, M.A., B.Sc, M.D., Professor of Pathology in the University 

 of Sydney, and H. G-. Chapman, M.D., B.S., Demonstrator of Physiology 

 in the University of Sydney. 



(Communicated by Dr. C. J. Martin, F.K.S. Eeceived April 30, — Kead 



May 31, 1906.) 



(From the Physiological and Pathological Laboratories of the University of Sydney.) 



In suitable conditions outside the animal body the most obvious result of 

 the interaction of a precipitin antiserum with its homologous proteid is the 

 appearance of a flocculent precipitate in previously clear solutions. The 

 accepted explanation is that the antiserum contains a precipitating substance 

 or precipitin, through tbe agency of which the precipitable substance, repre- 

 sented by the homologous proteid, is thrown out of solution. It would 

 generally be admitted that the precipitin to some extent enters into the 

 constitution of the precipitate, since it is through combination with definite 

 quantities of precipitin that the proteid is by most observers supposed to be 

 rendered insoluble. Nevertheless, even if that view of the interaction be 

 adopted, and still more if the precipitin be regarded as the analogue of 

 a ferment or as a non-proteid body altogether, it has been generally taken 

 for granted that the precipitate is constituted essentially by the precipitation 

 of the test proteid, and this conception of the source of the " precipitable " 

 substance has dominated all hypotheses and statements regarding the nature 

 of precipitin reactions. 



So far, however, as our observations go, we are led more and more definitely 

 to the conclusion that the great bulk of the substance that is thrown out of 

 solution and that goes to form the precipitate, is supplied by the antiserum 

 and not by the test proteid, and that, whatever part in the interaction is 

 played by the test proteid, it is not merely that of supplying a substance to 

 be thrown out of solution by the precipitin antiserum. 



That the antiserum itself is the main source of the " precipitable " 

 substance is indicated by the fact that quantities of the test proteid many 

 times too minute to yield an appreciable precipitum with ordinary proteid 

 precipitants may still yield distinct precipitates on the addition of a suitable 

 antiserum. Even if the whole of the test proteid were thrown out of 

 solution, it would be altogether insufficient to account for the mass of the 



