necessary for the production of the bactericidal action must be supplied by
the blood of the patient treated. This latter complement may not suit the
immune body, that is, may not be fixed to the bacterium by means of it, or
if the latter event does occur, may fail to bring about the death of the
bacteria. These circumstances serve, in part at least, to explain the fact
that the success attending the use of anti-bacterial sera has been much
inferior to that in the case of antitoxic sera.

[Sidenote: (b) Agglutination.]

Another property which may be possessed by an anti-bacterial serum is that
of agglutination. By this is meant the aggregation into clumps of the
bacteria uniformly distributed in an indifferent fluid; if the bacterium is
motile its movement is arrested during the process. The process is of
course observed by means of the microscope, but the clumps soon settle in
the fluid and ultimately form a sediment, leaving the upper part clear.
This change, visible to the naked eye, is called _sedimentation_. B. J. A.
Charrin and G. E. H. Roger first showed in the case of _B. pyocyaneus_ that
when a small quantity of the homologous serum (_i.e._ the serum of an
animal immunized against the bacterium) was added to a fluid culture of
this bacillus, growth formed a sediment instead of a uniform turbidity.
Gruber and Durham showed that sedimentation occurred when a small quantity
of the homologous serum was added to an emulsion of the bacterium in a
small test-tube, and found that this obtained in all cases where Pfeiffer's
lysogenic action could be demonstrated. Shortly afterwards Widal and also
Gruenbaum showed that the serum of patients suffering from typhoid fever,
even at an early stage of the disease, agglutinated the typhoid bacillus--a
fact which laid the foundation of serum diagnosis. A similar phenomenon has
been demonstrated in the case of Malta fever, cholera, plague, infection
with _B. coli_, "meat-poisoning" due to Gaertner's bacillus, and various
other infections. As regards the mode of action of agglutinins, Gruber and
Durham considered that it consists in a change in the envelopes of the
bacteria, by which they swell up and become adhesive. The view has various
facts in its support, but F. Kruse and C. Nicolle have found that if a
bacterial culture be filtered germ-free, an agglutinating serum still
produces some change in it, so that particles suspended in it become
gathered into clumps. E. Duclaux, fo