298 TUBERCULOSIS 



Petroff's method is also recommended as giving satisfactory results. In 

 this, sputum is shaken with an equal volume of 3 per cent, caustic soda 

 solution, and the mixture is placed for half an hour in the incubator at 

 37° C. At the end of this time it is made neutral to litmus with hydro- 

 chloric acid and then centrifuged. Some of the deposit thus obtained is 

 then planted on egg medium to which gentian violet has been added iu 

 the proportion of 1 : 10,000, the dye having an inhibitory action on the 

 growth of various organisms. A pure culture of the tubercle bacillus is 

 often obtained. ' 



Another method is that introduced by Twort ; portions of sputum are 

 exposed to the action of a 2 per cent, solution of ericolin (a glucoside) for 

 an hour at 38° C. , and thereafter cultures are made on Dorset's medium. 



(4) Reactive pfienomena. — The presence of immune-substances in the 

 blood and the tuberculin reaction, along with the methods of applying the 

 respective tests, have been described above (p. 293). 



