e to inject sputum from a phthisical person into the lower animal
and tuberculosis follows, and then announce to the profession that we
have demonstrated the relation of the cause and effect between bacilli
and phthisis? Why we would start such an uproar of objections as would
speedily convince us that there was much work yet in the domain of
bacteriology.

The scientific investigators would say you have injected with the
sputum into the blood of your unfortunate patient, pus, morphological
elements, and perhaps half a dozen other forms of bacteria, any one of
which is just as likely to produce the disease as the bacillus you
have selected.

The first important step is, first isolate your bacillus. If I were to
take a glass plate, one side of which is coated with a thick solution
of peptonized gelatin, and allow the water to collect, the gelatinous
matter will become solid. If now, with a wire dipped in some
tuberculous matter, I draw a line along the gelatin, I have deposited
at intervals along this line, specimens of tubercle bacilli. If this
plate be now kept at a proper temperature, after a few days, wherever
the bacilli have been caught, a grayish spot will appear, which,
easily seen with the naked eye, gradually spreads and becomes larger.
These spots are colonies containing thousands of bacilli. Let us
return to our gelatin plate.

We find a spot which answers to the description of a colony of
tubercle bacilli. We now take a minute particle from this colony on a
wire and convey it to the surface of some hardened blood serum in a
test tube. We plug the tube so that no air germs may drop in, and
place it in an incubator at the proper temperature. After several
days, if no contamination be present, a colony of bacilli will appear
around the spot where we sowed the spores. Let us repeat the process.

Take a particle from this colony, and transfer it to another tube.
This is our second culture. This must be repeated until we are
satisfied that we have secured a _pure_ culture. If this be carried to
the twenty-fifth generation, we may be assured that there remains no
pus, no ptomaines, nothing but the desired bacilli.

It is a proper material now for inoculation, and if we inoculate some
of the lower animals, for instance the monkey, we produce a disease
identical with phthisis pulmpnalis. Bacteria also afford peculiar
chemical reactions. For example, nitric acid will discharge all the
color from all bacilli arti