232 PATHOGENIC 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. 



