180 



The advantage of this method over that of comparison with 

 standard suspensions is that it takes advantage of a unit which 

 is always available, because it only requires persons with normal vision, 

 while standard suspensions may alter in the course of time, and 

 may be lost. 



The method however can only be applied to suspensions which 

 are of such a constitution that the letters to be read through them 

 preserve their sharp contour right up to the moment they com- 

 pletely disappear. If on the contrary the contours gradually be- 

 come more and more indistinct the method cannot be used. This 

 happens when the individual particles in the suspension are di- 

 stinctly larger than an isolated bacterium, for example, in spon- 

 taneously agglutinating suspensions. Doubtless the indistinct con- 

 tours arise from the comparatively large particles diffracting a 

 considerable amount of light rays at small angles, while the light is 

 deflected by sufficiently small particles more uniformly in all di- 

 rections, which envelops the letters in a diffuse white haze. 



The concentration 0.4 used in the agglutination experiments, is 

 comparatively large. It has been found that this produces more 

 rapid and obvious agglutination than is the case with thinner sus- 

 pensions, i 



To obtain further insight into the agglutination conditions the 

 simple test must be supplemented by absorption tests. 



As an orientation experiment for the most suitable technique 

 for the absorption test, I first arranged some tests with homologous 

 absorption at different temperatures and for different periods. A 

 suspension of a concentration about 2 of strain I 6 was prepared. 

 To 20 parts of this was added 1 part of the corresponding immune 

 serum and the mixture was then divided into 6 portions, which 

 were allowed to absorb at 4°, 17° and 37°, half of them for2 hours 

 and the other half for 6 hours. The tubes were centrifuged and an 

 agglutination test carried out with the supernatant fluid on a suspen- 

 sion of I 6. At 17° and at 37° both for 2 hours and 6 hours the 

 absorption brought down the titre to 1 : 25, while in the cold it was 

 1:50 after 2 hours, and 1:25 after 6 hours but with partial agglu- 

 tination in 1:50. 



In another experiment in which two different sera were absorbed 

 with their homologous strains in so small a concentration that a 

 little of the agglutinin was left, it was shown that most of the 

 absorption had taken place after 15 minutes but that it still went 

 on slightly for from 2 1 / 4 — 7 hours. 



An experiment with homologous absorption where the serum 

 concentration as well as the bacterial concentration was varied, 

 showed that a decrease of the former or an increase of the latter, 

 increased the absorbed fraction of agglutinin. It was even more 

 marked than Eisenberg & Volk found. If it is desired to give the 

 absorption specially favourable conditions for its action we must 

 therefore increase the bacterial concentration and diminish the serum 

 concentration. It was also noted in the same experiment that ab- 



