206 BACTERIA. 



almost as exhaustively in the daily papers as it is in special 

 treatises and in the medical journals. It has long been known 

 that tuberculosis was an inoculable disease, but it was only 

 quite recently (1883) that Koch and his pupils were able to 

 demonstrate that a specific organism could be separated from 

 tuberculous tissue and cultivated outside the body the 

 cultivated organism having all the characters of the organism 

 found in the tissues and that when introduced into certain 

 animals, this organism was capable of producing tubercular 

 disease, the organism in turn being again demonstrable in 

 the new tubercular growth. 



It was for long found to be an exceedingly difficult matter to demonstrate 

 any specific micro-organisms in tubercular tissues by means of aniline or 

 other nuclear stains and Baumgarten's method x was introduced after he 

 had failed to attain his object by any of the ordinary methods. The 

 difficulty he had, however, was that, although the tubercle bacilli un- 

 doubtedly resisted the potash solution, other organisms were also more 

 resistant than were the animal tissues, so that there was no great differen- 

 tiation except in size and form between the tubercle bacillus and other 

 bacteria. While Baumgarten was working out his method, Koch had 

 completed a series of investigations, the outcome of which was that he 

 proved that by the addition of a small quantity of an alkali to the aniline 

 stain the dye was rendered capable of penetrating the resistant outer 

 membrane of the tubercle bacillus. It was afterwards found that aniline, 

 thymol, turpentine, or carbolic acid, added to the stain, bring about 

 the same results ; these substances, acting, apparently, as mordants on 

 the tissues. The next step in the process of demonstration of the tubercle 

 bacillus was taken when it was found that this organism differed from 

 others in the fact that it retained the staining reagent most tenaciously, 

 even the strong mineral acids, which readily discharge the colouring matter 

 from nuclei and other bacteria having little effect, if acting for a short time 

 only, in taking out the stain from tubercle bacilli ; in sections stained in an 

 aniline colour mixed with one of these substances and then treated with 

 an acid, a most beautiful differential staining was obtained, the stained 

 bacilli standing out most prominently from the unstained tissues. 



Tubercle bacilli when stained are seen as delicate rods or 

 threads 1.5 to 3.5^ in length and about .21* in thickness, though 

 these dimensions are by no means constant even in the same 

 preparation, and under different conditions the variations in 

 size are sometimes very marked. As in the case of anthrax 

 and cholera bacilli, the methods of staining and preparation 

 exert a marked influence in determining the apparent size 



1 The sputum, after being dried on a cover glass and then heated to 

 coagulate the albumen, was simply soaked in a solution of caustic potash. 

 Sections were treated in the same manner. 



