176 ELEMENTS OF WATER BACTERIOLOGY 



common practice in this country is to incubate litmus 

 milk tubes for 48 hours at 37° and then heat to boiling. 

 Tubes which have not coagulated spontaneously fre- 

 quently do so on heating. Biffi (1906) has pointed out 

 that milk to be used for this purpose should not be 

 sterilized in the autoclave. Temperatures above 100° 

 so alter the milk as to make its coagulation much slower. 

 Konrich (19 10) concluded from his experiments that 

 the coagulation of milk by the colon bacillus is often not 

 due to acid production, but to the secretion of a specific 

 coagulating ferment, since he found that the addition of 

 an amount of acid similar to that produced by the 

 organism failed to coagulate. 



The production of indol in a peptone solution is an- 

 other test very generally used in this country and in 

 England as diagnostic of " typical " B. coli. The 

 usual procedure has been to incubate a tube of an 

 aqueous solution containing i per cent peptone and 

 .01 per cent sodium nitrite for four days at 37° and to 

 test for indol by adding i c.c. of a .02 per cent solution 

 of sodium or potassium nitrite and i c.c. of a i to i 

 solution of sulphuric acid. Both the tube and the 

 reagents should be cooled on ice before mixing, and the 

 tube should be left in a cool place for an hour afterward 

 to allow time for the characteristic rose-red color of 

 nitroso-indol to develop. 



Marshall (1907) and other German and English work- 

 ers have shown that this nitrite and sulphuric acid test 

 for indol often gives incorrect 'results and that the test 

 modified by Bohme from Ehrlich is both more sensitive 

 and more accurate. Two solutions are used; No. i is 



