SPECIFIC BACTERIAL REACTIONS 169 



dilutions to use a solution of approximately 0.25 per 

 cent, phenol in normal saline. It is advisable to make 

 up fresh such dilutions about once in two weeks, as the 

 phenol may cause some coagulation of the tuberculin, 

 resulting in precipitation and deterioration. Dilu- 

 tions showing marked sedimentation should be dis- 

 carded. 



New Tuberculin (" T. JR."). — Realizing that old 

 tuberculin was merely a toxin of tubercle bacilli, and 

 that the immunity produced by it was to the toxin only 

 and not to the bacterium itself, Koch, in 1897, seven 

 years after the announcement of " O. T.," described 

 a new tuberculin. This consisted of the residue 

 (Rilcktand) of tubercle bacilli and for brevity is 

 known as " T. R." This tuberculin is prepared by 

 drying in vacuo a virulent culture of tubercle bacilli. 

 The dried substance is powdered in a mortar and 

 triturated with normal saline. After centrifugation 

 the supernatant cloudy fluid is discarded. The resi- 

 due is again dried, ground, extracted with salt solu- 

 tion and centrif uged. This time the clean supernatant 

 fluid is pipetted off and retained. The process is re- 

 peated until the residue is entirely used up. The clear 

 solutions are united and added to glycerin, forming a 

 twenty per cent, solution. ** T. R." is standardized 

 so that one cubic centimetre represents ten milli- 



