474 JOHN F. NORTON AND MARY V. SAWYER 



of indol by Bad. typhosum by an oxygen current. Our cultures 

 were incubated aerobically, except in the case of the strict 

 anaerobes. 



INDOL REAGENTS 



Numerous tests for indol have been suggested and used. 

 Nelson (1916) gives four: (1) dimethylamine, glycolic acid, 

 glyceric aldehyde and sulphuric acid, giving a pink color; (2) per- 

 uvic aldehyde, sulphuric acid and ferric sulphate, giving a violet 

 color; (3) vanillin and an acid, giving an orange color; (4) 

 Salkowski test — sulphuric acid and potassium nitrite, giving a 

 pink to red ring. Escallon (1908) recommends furfural. This, 

 in the presence of hydrochloric acid, gives an orange yellow 

 color. It is claimed that this test is sensitive to 1 part in 800,000. 

 Baudisch (1915) describes a reaction using nitromethane. By 

 far the most satisfactory test is that suggested by Ehrlich (1901). 

 The reagent is prepared by dissolving 4 grams of paradimethyl- 

 amido-benzaldehyde in 380 cc. of alcohol and adding 80 cc. of 

 concentrated hydrochloric acid. A red color is formed in the 

 presence of indol at the junction of the reagent and the liquid to 

 be tested if the former is added so that it forms a layer on top. 

 A solution of potassium persulphate is sometimes added to bring 

 out the color more clearly but we have found the reagent quite 

 satisfactory without this. In making our tests, if a red color 

 appeared on adding the Ehrlich reagent, 1 cc. of amyl alcohol 

 was added and the tube shaken. The red coloring matter, if 

 due to indol, is soluble in amyl alcohol. 



SUMMARY 



In the following table we have summarized the results of our 

 tests together with those we have been able to find in the Uter- 

 ature. Owing to the uncertainty of results obtained by use of 

 the older methods of testing for indol, only relatively recent work 

 has been included. The organisms are divided into two groups : 

 (I) those which may pretty definitely be regarded as giving 

 negative tests and (II) those for which positive results have 

 been reported. It should be noted that in every instance where 



