142 ESSAYS ON BACTERIOLOGY. 



the bacilli are somewhat altered in shape, and perhaps 

 do not resist strong decolorizing agents as well as those 

 colored by a slower process. The best results are got 

 by allowing the specimens to lie in the cold carbol- 

 fuchsin for twelve hours or more. If the bacilli are 

 few and therefore difficillt to find, several methods are 

 available for concentrating them. Pure liquid car- 

 bolic acid added to sputum coagulates the parts most 

 likely to contain the bacilli, thus rendering the selec- 

 tion easier. If sputum be diluted with five or ten 

 times its volume of water containing a small per cent, 

 of potassium or sodium hydrate, and boiled, it be>- 

 comes liquefied and may be set aside for a day or two 

 to settle in a conical glass. The sediment may now 

 be examined with more certainty of finding the bacilli. 

 Or the sputum, thus liquefied or simply shaken with 

 water, may be rapidly precipitated with the centrifii- 

 gal machine. The microscopic examination of the 

 specimen thus prepared may- at once reveal the tuber- 

 cle bacilli, or may require great care and painstaking, 

 involving considerable time and the study of a num- 

 ber of specimens obtained at different times. Only 

 positive results are of definite significance. The find- 

 ing of tubercle bacilli stamps the case as tuberculosis. 

 A negative result is only of relative significance, its 

 value depending upon circumstances and upon the 

 thoroughness and skill of the examination. It does 

 not absolutely exclude tuberculosis, since the bacilli 

 may not have been present in the material examined 



