286' BACTERIOLOGICAL EXAMINATION OF WATER. 



Potato. A large potato is well washed and scrubbed with a 

 brush. A cylinder is then bored from its interior and cut 

 obliquely. The brown peel is cut oft' from the ends, and the 

 wedges so obtained are allowed to soak overnight in distilled 

 water so as to get rid of the excess of starch. Each wedge is 

 then placed in a potato tube previously fitted with a pad of 

 wool at the bottom and filled for about an inch in depth with 

 distilled water. The tubes are plugged with cotton wool 

 and then sterilised in Koch's steamer for one hour. Before 

 use the potato tube should be incubated at 37 C. for twenty- 

 four hours, in order to make sure that sterilisation has been 

 affected. Sometimes it is necessary to autoclave the medium, 

 but this should be avoided if possible. If sterilisation for 

 one hour at 100 C. renders the potato too soft, discontinuous 

 sterilisation for twenty minutes on three successive days should 

 be practised. 



Milk. Fresh milk is steamed for fifteen minutes in the Koch's 

 steriliser, and placed in a cool place over-night to facilitate 

 separation of the cream. The milk is then siphoned off from 

 beneath the cream and placed in sterile test-tubes. The tubes 

 are plugged and sterilisation effected by steaming at 100 C. for 

 twenty minutes on three successive days. For cultivation of the 

 B. enteritidis sporogenes whole milk should be placed in sterile 

 tubes and then sterilised as above. 



Litmus-whey. This medium was suggested by Petruschky 

 for the study of the amount of acid produced by the fermenta- 

 tion of lactose. Fresh milk is warmed, and the casein coagu- 

 lated by means of a little hydrochloric acid. The separated 

 casein is then filtered off' and the clear whey neutralised with 

 dilute sodium hydrate solution. The ffuid is then steamed 

 for two hours ; a little acid albumen usually separates and is 

 filtered off*. The filtrate should now be clear, colourless, and 

 neutral in reaction. To the filtrate 5 per cent, of a saturated 

 alcoholic solution of litmus is added, and the medium placed in 

 tubes and sterilised. For accurate work it is better to add 

 neutral litmus to the neutral whey, and then sterilise as usual. 



Lactose Litmus Solution. This medium is made as follows : 

 Dissolve two grammes of lactose in 40 c.c. of broth and add 

 water to make up to 100 c.c. Carefully neutralise and boil in 



