258 THE POPULAR SCIENCE MONTHLY. 



active middle age are carried off by the same cause. Prior to Koch it 

 had been placed beyond doubt that the disease was communicable ; 

 and the aim of the Berlin physician has been to determine the precise 

 character of the contagium, which previous experiments on inocula- 

 tion and inhalation had proved to be capable of indefinite transfer 

 and reproduction. He subjected the diseased organs of a great 

 number of men and animals to microscopic examination, and found, 

 in all cases, the tubercles infested with a minute, rod-shaped para- 

 site, which, by means of a special dye, he differentiated from the 

 surrounding tissue. It was, he says, in the highest degree impres- 

 sive to observe in the center of the tubercle-cell the minute organ- 

 ism which had created it. Transferring directly, by inoculation, the 

 tuberculous matter from diseased animals to healthy ones, he in 

 every instance reproduced the disease. To meet the objection that it 

 was not the parasite itself, but some virus in which it was imbedded 

 in the diseased organ, that was the real contagium, he cultivated his 

 bacilli artificially, for long periods of time, and through many succes- 

 sive generations. With a speck of matter, for example, from a tuber- 

 culous human lung, he infected a substance prepared, after much trial 

 by himself, with the view of affording nutriment to the parasite. Here 

 he permitted it to grow and multiply. From this new generation he 

 took a minute sample and infected therewith fresh nutritive matter, 

 thus producing another brood. Generation after generation of bacilli 

 were developed in this way without the intervention of disease. At 

 the end of the process, which sometimes embraced successive cultiva- 

 tions extending over half a year, the purified bacilli were introduced 

 into the circulation of healthy animals of various kinds. In every case 

 inoculation was followed by the reproduction and spread of the para- 

 site, and the generation of the original disease. 



Permit me to give a further, though still brief and sketchy, account 

 of Koch's experiments. Of six Guinea-pigs, all in good health, four 

 were inoculated with bacilli derived originally from a human lung, 

 which, in fifty-four days, had produced five successive generations. 

 Two of the six animals were not infected. In every one of the in- 

 fected cases, the Guinea-pig sickened and lost flesh. After thirty-two 

 days one of them died, and after thirty-five days the remaining five 

 were killed and examined. In the Guinea-pig that died, and in the three 

 remaining infected ones, strongly pronounced tubercular disease had 

 set in. Spleen, liver, and lungs were found filled with tubercles ; while 

 in the two uninfected animals no trace of the disease was observed. 

 In a second experiment, six out of eight Guinea-pigs were inoculated 

 with cultivated bacilli, derived originally from the tuberculous lung 

 of a monkey, bred and rebred for ninety-five days, until eight genera- 

 tions had been produced. Every one of these animals was attacked, 

 while the two uninfected Guinea-pigs remained perfectly healthy. 

 Similar experiments were made with cats, rabbits, rats,, mice, and other 



