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kept it to himself, satisfied himself that this was a con- 

 stant, not an occasional or accidental association ; that 

 the same bacteria were present in the spontaneous tuber- 

 culosis of animals the hog, chicken, ape, guinea-pig, 

 and rabbit ; then he devised, by experimentation, a proper 

 medium, solid of course, for cultivating the organisms 

 outside of the animal body under constant microscopic 

 supervision, comparing them with fresh bacilli from tu- 

 berculous tissues ; satisfied himself again by personal 

 experiment of the inpculability of tuberculosis ; found 

 that while vaccination of the rabbit or guinea-pig with 

 fresh tuberculous matter induced the disease, inocula- 

 tion with such material after lying in alcohol for a 

 month or dry for two months, was impotent to cause the 

 disease, and contained no living bacilli ; found that the 

 bacteria were often, not always, present in the sputum of 

 tuberculous patients, but never, so far as examined, in 

 that of others. Having thus made a preliminary investi- 

 gation, Koch proceeded to the experimentum crucis with 

 bacilli which had grown from the tuberculous tissue under 

 his eyes ; which were therefore proved to be the progeny 

 of the original ones, not by the theory of probabilities, 

 not simply by their identical size, shape, and chemical 

 reaction, but by the fact that he had seen them proceed 

 from the first as continuities of structure ; which were 

 seen under the microscope to be quite free from any 

 foreign solid matter, bacterial or other; which were 

 proven to be equally free from any foreign matter in 

 solution because growing in successive cultures upon 

 solid soil ; which had been carried from the first to the 

 eighth generation ; which had been thus isolated from 

 the original animal tissues three, four, five, even six 

 months. With these isolated descendants of the bacilli 

 found in tuberculous tissues, Koch inoculated numerous 

 animals using over two hundred altogether not only 

 the susceptible rabbits and guinea-pigs, but also cats, a 

 dog, white rats which had resisted inoculation by injec- 

 tion and by feeding with tuberculous materials, and field 

 mice. Inoculation was made in the skin, the abdomen, 



