THE PRECIPITINS 



22g 



chemical substances. Precipitating sera should, therefore, be 

 kept in a dry state in a cool place, and preserved from light. 



Precipitoids appear to have a stronger affinity for the precipitate 

 substance than has the unaltered precipitin an alteration in 

 affinity similar to that which we have seen to occur sometimes in 

 the case of agglutinin, and which leads to the formation of what 

 we have termed pro-agglutinoid. Thus, if a serum which has 

 been heated until it has lost its precipitating power be mixed 

 with unheated precipitin, the mixture will not form any precipitate 

 after the addition of small amounts of the normal serum ; it is only 

 after enough of the latter has been added to combine with all the 

 precipitoid that a precipitate begins to appear. The same pheno- 

 mena may occur in working with old and degenerated sera, or 

 occasionally even with fresh ones. An example will make this 

 clear. 





Here the maximum precipitate was given when 8 parts of anti- 

 serum were mixed with i part of normal serum. When i part of 

 normal serum was mixed with 24 parts of antiserum, there was no 



