142 ATLAS OF BACTERIOLOGY. 



ler's reagent is reduced almost immediately to black 

 mercurial oxide. A strip of paper which has been 

 dipped in the reagent may be hung over bouillon 

 cultures, or the latter may be distilled after addition 

 of MgO and the distillate treated with Nessler's re- 

 agent. A yellow to reddish-brown color indicates the 

 presence of ammonia. Control tests must be made. 



6. Aromatic Metabolic Products. 



It is evident that albumin gives rise, under the in- 

 fluence of very many varieties of bacteria, to aromatic 

 bodies of which indol, skatol, phenol, and ty rosin 

 are the best known. Methodical investigations have 

 been made only in regard to indol and phenol, as 

 these bodies are easily recognized. 



Demonstration of indol : To the bouillon culture 

 which should not be less than a week old and made 

 without any addition of sugar about half its volume 

 of ten-per-cent sulphuric acid is added. If a rose to 

 bluish-red color appears forthwith on warming to 

 about 80, then indol and nitrite are both present, as 

 this nitrosoindol reaction requires the presence of 

 both bodies. The test is generally successful in 

 cholera and other vibriones and occasionally in diph- 

 theria (red cholera reaction). But as a general thing 

 the addition of sulphuric acid does not suffice, and it 

 is necessary to add a little nitrite. This may be done 

 later, after the culture has been heated without 

 nitrite, and no reaction or a very doubtful one has 

 been obtained. Of the solution containing about 

 0.05 per cent sodium nitrite we add 1 to 2 c.c. until the 

 maximum of the reaction is secured. The addition 

 of strong nitrite solutions gives the acid fluid a 



