232	PA THOGENIC BACTERIA.
abscesses invariably followed their introduction, whether
dead or alive, and nodular growths in the lungs were
constant sequelae of their injection into the circulation.
In such nodules the bacilli could be found unabsorbed
and unaltered. It seemed as if the fluids of the body
could not effect solution of the bacteria. The ineffectual
attempts at immunization, with the results given, probably
depend upon the inability of the tissues to take up from
the bacilli whatever immunizing substances they might
contain, first, because of the impossibility of dissolving
them, and, second, because the irritating powers they
possess interfere with the direct action of normal fluids
and uninjured body-cells, and always subject the bacteria
to semi-pathological conditions.
From these data, which he carefully studied out, Koch
concluded that it would be necessary to bring about some
artificial condition advantageous to the absorption of the
bacilli, and for the purpose tried the action of diluted
mineral acids and alkalies. The chemical change brought
about in this manner facilitated absorption, but the ab-
sorption of bacilli in this altered condition was not fol-
lowed by immunity, probably because the chemical com-
position of tubercle-toxin (or whatever one may name
the poisonous products of the bacillus) was changed by
the reagents used.
Tuberculin, with which Koch performed many experi-
ments, was found to produce immunity only to tubercu-
lin, not to bacillary infection.
Pursuing the idea of fragmenting the bacilli, or in some
way treating them chemically in order to increase their
solubility, Koch found that a 10 per cent, sodium hydrate
solution yielded an alkaline extract of the bacillus, which,
when injected into animals, produced effects similar to
those following the administration of tuberculin, except
that they were briefer in duration and more constant in
result. The marked disadvantage of abscess-formation
following the injections, however, remained. This fluid,
when filtered, possessed the properties of tuberculin.