78 BACTERIAL POISONS OF INFECTIOUS DISEASES. 



of tuberculin. At first he extracted the bacilli with a decinormal 

 solution of caustic soda. This extract, when filtered through paper 

 and injected into animals, induced a reaction similar to that which 

 followed the use of the original preparation ; but it was found that 

 abscesses were likely to follow the employment of this material. It 

 was also observed that it contained tubercle bacilli and for this 

 reason it was filtered through porcelain. The filtrate was germ-free, 

 but was found not to possess the reaction of tuberculin. It was thus 

 evident that filtration through porcelain had removed not only the 

 bacilli, but the active constituent of the preparation. In his further 

 researches with the bacillus, Koch was able to extract two character- 

 istic chemical substances, both of which belong to the unsaturated 

 fatty acids. One of these is freely soluble in dilute alcohol and is 

 readily saponified with alkalis, while the other is soluble only in 

 boiling absolute alcohol and saponifies with difficulty. Both of iJiese 

 substances take the so-called tubercle stain, i. e., they are stained in- 

 tensely red with carbolic fuchsin and retain this color on treatment 

 with dilute nitric acid and with alcohol. Bacilli from which these 

 fatty bodies have been extracted retain their form, but no longer 

 take their characteristic stain. 



Finally, Koch prepared his improved tuberculin in the following 

 manner : The bacilli of fresh, highly virulent cultures are dried and 

 rubbed into a very fine powder, which is then 'added to distilled 

 water with which it forms a colloidal mixture. This aqueous prep- 

 aration is placed in a centrifuge which makes four thousand revolu- 

 tions per minute, and after half an hour it separates into an upper, 

 opalescent but transparent layer, which contains no bacilli, and into 

 a deposit. The deposit is dried, again rubbed up in a mortar, mixed 

 with water and centrifuged, and this is repeated until there is 

 obtained a perfectly clear fluid. The upper layer obtained the first 

 time in the centrifuge Koch designated as TO, while the upper layers 

 obtained by subsequent treatment in the centrifuge he distinguished 

 as TR. The addition of 50 per cent, glycerin to TO causes no 

 change, while in TB it produces a flocculent white predpitate. This 

 indicates that TB contains those substances which are insoluble in 

 glycerin, while TO is made up of the constituents of the bacterial 

 cells soluble in this reagent. In its action upon man and other 

 animals, TO resembles the original tuberculin while TB is supposed 

 to have the curative action first attributed to tuberculin without any 

 of its ill-effects. Koch stated that with TB men and animals could 

 be so immunized that they did not react to TO or crude tuberculin. 

 TB is preserved with the addition of 20 per cent, of glycerin. 



De Schweinitz * isolated from liquid cultures of the bacillus tuber- 

 culosis a crystalline substance having a melting point of 161° to 

 164°, readily soluble in ether, alcohol and water. Analysis showed 

 ' Tranaactiom of the AssoeiaHon of American Physicicms, 1897. 



