IDENTIFICATION OF BACTERIAL SPECIES 5 
too great an application of either diminishes or destroys the immunizing power of the injection 
(14/1). The date on which the reaction first appears after injection is rather variable, and depends 
on two or three factors, although chiefly on the strengtli of culture. Thus a Guinea-pig, which I 
injected with living cholera culture on 18th April, i8g6, showed the reaction well on the 21st; 
whilst one injected with dead cholera culture on i6th March, 1896, did not exhibit marked 
agglutinative action till 23rd Marcli. It is not even necessary to inject the bodies of the bacteria : 
filtered cultures give similar but weaker or more delayed results (14/^, 9). By the simultaneous 
injection of two kinds of micro-organisms agglutinins for both can be produced in the same serum 
(14^). 
' Passive ' immunization is generally produced by the injection of serum from an actively 
immunized animal. In such a case the agglutinative property shows itself much sooner. Thus, 
in a Guinea-pig which I injected at 3 p.m. on 9th March, 1896, agglutinin could be detected 
in the blood already at 3.30 p.m., and had attained its maximum strength about 6.50 p.m., 
remaining at the same strength for about one week afterwards. An actively immunized Guinea- 
pig generally possesses agglutinative serum for a considerably longer period. 
The nature of the agglutinating substance has not yet been distinctly ascertained. Filtra- 
tion of the serum through a porcelain filter diminishes its power. Precipitation by neutral salts of 
the globulin of the serum or the fibrinogen of the plasma causes loss, or sometimes only 
diminution, of agglutinative power in the filtered fluid. Similarly serum is generally only two- 
thirds as powerful as plasma, so that the agglutinin seems to be in some way bound up with the 
proteids of the blood. Leucocytes, when shed, do not apparently give off any agglutinin. Tem- 
perature affects agglutinin in proportion to its height and the time it is allowed to act. Ten 
minutes at 75° C. destroys the substance, and an hour at 57° C. causes a diminution of effect. 
Merely heating to 60° C. does not deprive serum of its power (i, 14/'). 
What has been said hitlierto relates nearly entirely to animals, and of these, to Guinea-pigs. 
We shall now consider the question mainly in relation to Man. 
The serum of normal Guinea-pigs rarely exhibits agglutinative power, but that of some 
other animals, particularly of the horse, sometimes show it in a marked but varying degree. So, 
too, does the serum of Man. The blood of many individuals possesses a pronounced agglomerating 
action on several forms of micro-organisms ; but the quantity of each agglutinin is small, not 
showing itself when the serum is diluted more than eight times {ja). Thus normal human serum 
presents some striking differences from that of immunized Guinea-pigs, principally in its multiple 
action. We shall consider later whether this is due to the presence of a common agglutinin 
or small quantities of each special agglutinin. 
When normal serum lias any agglomerating action it is often nearly equal on at least three 
kinds of bacteria — B. typhosus, the cholera vibro, and B. col'i communis. In one diseased condition 
marked by the evident presence of abnormal substances in the serum, namely, jaundice, the 
agglutinating action is nearly always present ; and if the serum be not diluted, it is as rapid and 
complete as with the serum of an immunized animal. Neither normal nor icteric serum, 
however, possesses the power of inhibiting movement. For this reason it seems probable that 
