236 22 



At Ihe same lime llie agglutination observed in set D shews IhaltheO'Ol percent 

 of horse-serum may have no recognisable influence upon the agglutinating action 

 of the large or the small amounts of acid, but may yet impede the action of the 

 intermediate quantities, thus increasing the extent of the already more or less well- 

 marked "zone of inhibited agglutination" that so commonly occurs in the tubes 

 that contain the intermediate amounts of acid. 



The fact that the inhibiting influence of normal horse-serum upon the agglut- 

 ination caused by the acid does not follow the laws of simple proportion is evident 

 from the results given in columns 13, C, andD; for while the addition of 01 percent 

 of the serum diminishes the action of a given quantity of acid to about '-/a of its 

 usual value, the addition of 10 per cent of the serum diminishes it to only 

 one half. 



Proceeding now to give a short account of the agglutination by imnnine- 

 sei'uni of B. coli emulsion acidified by the addition of various quantities of acid, 

 we have found, as experiments already detailed would lead one to expect, that the 

 agghitinabiliti] exhibited by such acidified emulsions is extremely sensitive to minute 

 variations in the amount of acid they contain. Thus it appeared that the addition 

 of HCl in the proportion of 003 milligrams to each test-tube containing 15 c.cni. 

 Vi normal emulsion — a quantity of acid that would produce a just visible 

 agglutination if acting by itself alone — did not recognisably affect the agglutination 

 caused by the immune-serum; 006 mgrm. of the acid, which by itself would cause 

 strong agglutination, may diminish the agglutinability of the suspension by serum 

 to '/i5 of the value it gives when not acidified ; while both 024 and 048 mgrms. 

 of acid, each of them quantities that would produce agglutination if acting alone, 

 depressed the agglutinability to Vaa. Hence acidification of the emulsion makes it 

 much less easily agglutinated by the immune-serum. At the same time it makes 

 the whole course of the agglutination liable to irregularities; and causes Ihe occur- 

 rence of "zones of inhibition", and of paradoxical agglutination, in which a single 

 tube at some point in a set may exhibit marked agglutination, while its immediate 

 neighbours on either side show none. It is important to note that if the B. coli 

 emulsion in which the acidification has prevented the occurrence of agglutination 

 be suitably neutralised with NaOH solution, then agglutination ensues in a normal 

 manner; this shews that there can be no question here of a destruction of the 

 agglutinable group in the bacteria by the acid, similar to that which Eisknbeug 

 & Volk have assumed to lake place in the case of B. typhosus. 



So that the presence of these small quantities of HCl — (pianlities of 003 to 

 0'48 mgrms. of the acid in the 25 c.cni. of immune-serum, normal saline, and bac- 

 terial suspension made up as already described — that would cause agglutination 

 if acting alone, suffices to inhibit the agglutinating action of from 2 to 33 times 

 the amount of immune-serum that produces agglutination when acting by itself. 

 And no agglutination at all may be produced in tubes that contain enough 



