8 BACTERIOLOGICAL EXAMINATION OF WATER. 



water from the Elbe, taken below Hamburg and Altona, showed 

 that the greatest number of bacteria was obtained when O'l per 

 cent, of sodium carbonate was added to ordinary slightly alka- 

 line gelatine-peptone. Dahmen, working with water from the 

 Rhine, found that the addition of 0*15 per cent, of sodium car- 

 bonate to ordinary gelatine caused the development of the 

 greatest number of micro-organisms. My own experiments with 

 the South Hants water supply showed that more bacteria were 

 obtained on alkaline than on acid media. Two batches of 

 ordinary gelatine-peptone were prepared, one rendered just 

 alkaline to phenol-phthalein, and the other to litmus. To the 

 first batch N acid was added to the extent of 1*5 per cent., as 

 recommended by the American committee, and to the second 

 O'l per cent, of sodium carbonate as suggested by Reinsch. 

 Gelatine plates were then made with 0*5 c.c. of the South Hants 

 water and incubated at 22 C. The following average results 

 were obtained : 



1 c.c. South Hants water. Acidified Gelatine. Alkaline Gelatine. 



(1'5 per cent. N acid.) (0*1 per cent, sodiiirn 

 carbonate.) 



After 48 hours incubation at 22 C. ... 59 Colonies. ... 81 Colonies. 



72 ... 88 ... 96 



On the fourth day the plates were usually destroyed by the 

 liquefying organisms present in the water. All the water-organ- 

 isms usually found in the alkaline media appeared to grow in 

 the acid media, only a little more slowly. Hesse and Niedner 

 recommend the following medium for the bacteriological exami- 

 nation of water : 



Agar-agar . . . . '. . 1 '25 per cent. 

 Albumose (Hey den) . . . . . 0'75 

 Distilled water 98'00 



This medium requires no correction for acidity or alkalinity, and 

 permits the development of the greatest number of water- 

 organisms. The authors consider that water-plates for the 

 simple enumeration of colonies should be kept under observation 

 for three weeks, as numerous experiments have shown them that 

 at 20 C. in the first three days 30 per cent., in the first five days 

 about 70 per cent., and in the first ten days about 90 per cent. 



