238 24 



obtained are set down in Tables XXV to XXXII, Series 79 to 90. The figures in 

 Series 81, Table XXVI, where the agglutinating power of the serum treated by acid 

 was measured upon an agar-culture suspension of B. coli, give evidence of the fact 

 that extremely minute quantities of HCl — from 00()()5 to 00007 per cent — do not 

 recognisably alter the agglutinating power of the immune-serum; while amounts 

 varying from 0001 to 0.01 per cent of acid usually make the serum more actively 

 agglutinative than it was before the addition of the acid was made — but in these 

 particular experiments it not infrequently happens that irregularities and "zones of 

 inhibition" occur to interrupt the steady course of the agglutination. Yet such 

 irregularities are not due directly to the amount of the acid which has been added, 

 for they do not occur in the sets of tubes containing from 00005 to 00007 or from 

 00025 to 0015 per cent of acid; but they are often seen in the sets of tubes con- 

 taining intermediate amounts of acid, namely from 0001 to 0002 per cent. To give 

 a single instance of the intensifying influence of these small amounts of acid, the 

 figures in Series 81 prove that the presence of 000013 mgrni. of HCl in the immune- 

 serum added to a tube can increase its agglutinating power fourfold. 



The addition of somewhat larger amounts of HCl alters the agglutinating 

 power of the serum in a different way. A B. coli-agglutinating serum, diluted 

 1 to 100, and acidified with HCl to 2-439 per mille (i. e. to about Vis normal HCl) 

 as in Series 81, produces no agglutination in 15 c.cm. agar suspension (diluted to 

 2'5 c.cm.) when added in amounts ot from 001 to 000085 c.cm., although these 

 (piantitics contain from 25 to 213 agglutinating units; but at the same time, from 

 000085 to 000012 c.cm. of this acidified serum will produce nearly total agglutina- 

 tion, though 000012 c.cm. represents onlj' Va of an agglutinating unit. 



Thus it is clear that when the agglutinating power of the imnmne-serum thus 

 treated with acid is measured on an agar-culture suspension of B. coli, there are two 

 limiting values to choose from, a higher one that proves the serum to be weaker than, 

 and a lower one that proues it to be about three times as strong as, it was before il 

 was treated with the acid. The question now arises — which of these two values, 

 if either, correctly represents the process that is being measured? 



If the quantity of acid present in the different test-tubes that are agglutinated 

 to the same degree as the tube chosen as standard be calculated, it is found that 

 they all contain more or less the same amount of acid, whether the agglutinating 

 power appear to be diminished or increased, and although the various sera used 

 have been treated with widely differing quantities of acid and are present in widely 

 varying amounts. The numbers given in Series 81, for example, shew that in the 

 several sets of test-tubes in which the differently acidified sera — which contain 

 from 0-408 to 2493 parts per mille of HCl — have been added, the lower Hmit of 

 agglutination to the standard degree always occurs at the tubes containing from 

 0029 to 002() mgrm. HCl, while the amount of the agglutinating scrum in these 

 tubes varies between 000012 and 0007 c.cm. 



