40 WATER 



used, there is considerable danger that some of the other bacilli 

 present will multiply at a greater rate than any typhoid bacilli 

 present, and the latter be relatively less numerous after incuba- 

 tion, if not actually suppressed. Numerous chemical substances 

 have been tried, or advocated, as enabling the typhoid bacillus to 

 multiply relatively better than competing bacilli, but most of 

 them, on further investigation, have not been found to be reliable. 

 Malachite green has been found to be of some service, but a 

 series of different dilutions must be employed. The water is 

 placed in flasks, and sufficient malachite green broth is added to 

 each to make the strength of malachite green in the mixture I in 

 2000, i in 5000, I in 10,000 respectively. After incubation at 

 37 C., or preferably at 40 to 42 C., for 24 hours, the mixtures 

 are plated upon the solid media described below. 



On the whole it may be said that enrichment and selective 

 enrichment methods are less satisfactory than sedimentation or 

 direct concentration. 



(b) Concentration by mechanical precipitation by chemicals. 

 Chemicals are added which are harmless to typhoid bacilli but 

 which form a flocculent precipitate which carries down all or 

 most of the bacilli in the water and enables them to be obtained 

 in a small and easily handled bulk of material. If a centrifuge 

 is available precipitation is facilitated. 



A number of chemicals have been recommended for this 

 purpose, e.g. lead acetate and sodium hyposulphite (Vallet and 

 Schiider), iron sulphate (Picker), alum (Willson), liquor ferri 

 oxychlorati (Miiller). 



Picker's method is perhaps as good as any. In this method, 

 two litres of the water are placed in a tall cylinder, and mixed 

 with 8 c.c. of a 10 per cent, soda solution. Seven c.c. of a 10 per 

 cent, solution of ferrous sulphate are then added, and stirred in 

 with a glass rod. The mixture is placed in an ice-chest, and 

 allowed to stand for a few hours ; if a centrifuge is available the 

 precipitate can be separated at once. The precipitate is trans- 

 ferred to a sterile tube, and about half its volume of a 25 per cent, 

 solution of neutral potassium tartrate is added. The tube is 

 corked and well shaken, until the precipitate is completely 

 dissolved, more tartrate being added if necessary. From this 



