358 PUBLIC HEALTH BACTERIOLOGY 



which is more marked among the " typicals," which now 

 number 40 to 50 per cent only of the total coliforms. " This 

 means that the ratio or proportion between the typical B. coli 

 and the B. coli (both typical and non-typical) is altered during 

 the purification processes in the direction of reducing the 

 number of typical B. coli. This constitutes an intrinsic 

 difference between the bacterial flora of the raw and filtered 

 waters, and as the typical B. coli are considered specially 

 significant of undesirable excremental pollution, it is a 

 difference which may mean far more than the actual figures 

 seem to indicate. The processes which affect this modifica- 

 tion of the original biological attributes of the raw waters may 

 operate far more powerfully in the direction of eliminating 

 the microbes of epidemic disease " {Fifth Annual Report, p. 12). 

 It should also be noted that Houston uses glucose, lactose, 

 and saccharose gelatin media. His statement on this point is 

 as follows : " Particular attention must again be directed to 

 the value of these solid gelatin sugar media. Times without 

 number a microbe which failed to show any visible develop- 

 ment of gas in liquid sugar media, has been found to give 

 abundant gas when grown in solid gelatin media containing 

 the same sugars." First Report on Research Work, p. 7). 



These media are described on page 49 of the Report on the 

 Metropolitan Water Supply for January, 1907. They have 

 the following composition : 



Lemco . . . . . . 1 per cent. 



Peptone . . . . . . . . 2 ,, 



Gelatin . . . . . . . . 7*5 



KOH . . 1 c.c. of a 5 per cent solution. 



Add 1 per cent of glucose, lactose, or saccharose. The 

 glucose tubes are not tinted, the lactose tubes are tinted with 

 litmus, and the saccharose tubes with neutral-red. 



After inoculation, such tubes are placed for exactly three 

 hours in the blood-heat incubator. This melts the gelatin, 

 allows of some multiplication of the organisms to take place, 

 and helps to effect their distribution throughout the medium. 

 The tubes are then placed in the ice chest for half an hour to 

 allow the gelatin to set, and thereafter incubated for twenty- 

 four hours at 20-22 C. Abundant gas formation (in a posi- 

 tive result) is visible long before the twenty-four hours are 

 completed. 



In such a medium liquefaction of gelatin can also be noted. 



IV. Streptococci. To 10 c.c. of MacConkey's glucose 

 broth, quadruple strength, add 50 c.c. of sample water, and 



