1906.] -Main Source of " Precipitable " Substance, etc. 311 



sufficient to neutralise all the precipitin in O01 gramme of antiserum is, 

 roughly speaking, ten times greater in the case of the horse antiserum 

 (O0005 gramme, cf. Tuhe No. 3, Experiment I) than in the case of the egg 

 antiserum (0 - 00005 gramme, cf. Tube No. 2, Experiment III). These data 

 are not to be taken as accurately determining the minimum neutralising 

 amounts of homologous proteid for - 01 gramme of the antisera, as the 

 intervals above and below the critical points are too great. Nevertheless 

 they serve the purpose of indicating the nature of the difference between 

 the two antisera in question. They show that the precipitable content of the 

 egg antiserum is more easily discharged than that of the horse antiserum, 

 since it requires less of the homologous proteid for complete neutralisation 

 and precipitation. 



In estimating the capacity of an antiserum for interaction with its homo- 

 logous proteid, two factors, therefore, have to be considered : (1) the pre- 

 cipitable content of the antiserum, and (2) its precipitability. So far as our 

 observations go, these factors appear to be mutually independent. 



The results of operating with antisera whose precipitable content is not 

 easily discharged are of interest, since they indicate how erroneous 

 impressions may arise regarding the precipitation of the homologous proteid. 

 In such cases the precipitates may show a progressive increase with each 

 increase of the homologous proteid and only small precipitates with small 

 amounts of the proteid. 



In the readjustment of atom complexes that may take place within the 

 animal body when its proteid molecules repeatedly encounter alien proteid 

 molecules, as in processes of immunisation, it is not unreasonable to suppose 

 that the reaction on the part of the host results in the formation of 

 precipitable molecules which are endowed in certain circumstances with 

 many, in other circumstances with few, atom complexes capable of interacting 

 with corresponding atom complexes of the homologous proteid molecule. It 

 is possible that the precipitable content of such an antiserum is determined 

 by the number of precipitable molecules in a given molecule of the anti- 

 serum, while its precipitability is conditioned by the number of susceptible 

 atom complexes that each molecule possesses. 



The Role of the Homologous Proteid in Precipitin Reactions. 



Our observations do not enable us to decide whether the homologous 

 proteid is or is not diminished as a result of interaction with precipitin 

 antisera. We have, however, definite evidence that it is not wholly 

 exhausted and no conclusive evidence that it is sensibly exhausted even after 



