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.