418 BACTERIOLOGY. 
that by the aid of the presence or absence of this reac- 
tion in the serum of convalescents from suspected 
typhoid fever the nature of the disease could be deter- 
mined. It was further found if the serum of an animal 
thoroughly immunized to the typhoid bacillus was 
diluted with 40 parts of bouillon, and a similar dilu- 
tion made of the serum of non-immunized animals, 
and both solutions were then inoculated with a culture 
of the typhoid bacillus and placed in the incubator at 
37° C., that after the expiration of one hour macro- 
scopical differences in the culture could be observed, 
which increased in distinctness for four hours and then 
gradually disappeared. The reaction occurring is de- 
scribed as follows: In the tubes in which the typhoid 
culture is mixed with typhoid serum the bacilli are 
agglomerated in fine, whitish flakes, which settle to the 
bottom of the tube, while the supernatant fluid is clear 
or only slightly cloudy. On the other hand, the tubes 
containing mixtures of bouillon with cholera or coli 
serum, or the serum of non-immunized animals inocu- 
lated with the typhoid bacilli, became and remained 
uniformly and intensely cloudy. These serum mix- 
tures, examined microscopically in a hanging drop, 
show distinct differences.. The typhoid serum mixture 
inoculated with the typhoid bacilli exhibits the organ- 
isms entirely motionless, lying clumped together in 
heaps; in the other mixtures the bacilli are actively 
motile. 
These observations were made independently by 
Gruber and Durham, who maintained, however, that 
the reaction described by Pfeiffer was by no means 
specific, and that when the reaction is positive the 
diagnosis still remains in doubt, for the reaction is 
