66 MICRO-ORGANISMS IN WATER 



ised. But as the neutralisation of the larger quantity 

 of material is effected by a stronger (4 per cent.) solu- 

 tion of caustic soda, it is necessary, after multiplying 

 the amount of the 0-4 per cent, solution required for 

 so many cubic centimetres of material, to divide the 

 result by 10. For example, suppose 1 c.c. of broth 

 requires 0-25 c.c. of a 0*4 per cent, solution for its 

 neutralisation, 1,000 c.c. will require 250 c.c. of the 

 0-4 per cent, solution, or 25 c.c. of the 4 per cent, solu- 

 tion. By adopting this method the degree of alkalinity 

 can of course be far more carefully regulated and ac- 

 curately determined than by the litmus-paper test, and 

 it should be resorted to in those cases in which special 

 attention is to be directed to the numerical estimation 

 of colonies. On the other hand, in those numerous 

 cases in which comparative results only are desired — 

 e.g. in the examination of a water before and after 

 filtration or other treatment — the precise degree of 

 alkalinity of the medium is of less importance. 



Dahmen ^ has more recently investigated the effect 

 on the development of water bacteria of the degree of 

 alkalinity possessed by the culture medium employed. 

 Sodium carbonate was added in varying quantities to 

 the gelatine-peptone, and the result traced in the deve- 

 lopment of the bacteria present in water obtained from 

 the Rhine. It was ascertained that the addition of 0*15 

 per cent, sodium carbonate induced the development of 

 the largest numbers of water microbes. These results 

 confirm those previously obtained by Eeinsch in the 

 case of the river Elbe water. 



Dahmen^ has also conducted experiments on the 



^ ' Die bakteriologische Wassenmtersuchung,' Chemiker-Zeitungf 

 Jahrgang xvi., 1892, No. 49. 



^ * Die Nahrgelatine als Ursache des negativen Befnndes bei Unter- 

 suchung der Faeces auf Cholera-Bacillen,' Centralhlatt fur Bakteriologiey 

 vol. xii., 1892, p. 620. 



