46 METHODS OF CULTIVATION OF BACTERIA 



Two wedges are thus obtained, each of which is placed broad 

 end downward in a test-tube of special form (see Fig. 11). In 

 the wide part at the bottom of this tube is placed a piece of 

 cotton wool, which catches any condensation water 

 which may form. The wedge rests on the con- 

 striction above this bulbous portion. The tubes, 

 washed, dried, and with cotton wool in the bottom 

 and in the mouth, are sterilised before the slices of 

 potato are introduced. After the latter are in- 

 serted, the tubes are sterilised in the Koch steam 

 steriliser for one hour, or in the autoclave for 

 fifteen minutes, at 115 C. An ordinary test-tube 

 may be used with a piece of sterile absorbent wool 

 in its bottom, on which the potato may rest. 



Glycerin potato, suitable for the growth of the 

 tubercle bacillus, may be prepared by covering the 

 slices in the tubes with 6 per cent, solution of 

 glycerin in water, and steaming for half an hour. 

 The fluid is then poured off and the sterilisation 

 continued for another half-hour. 

 FIG. 11. Potatoes ought not to be prepared long before 



Ehrlich's being used, as the surface is apt to become dry 

 taini'ng piece an( ^ discoloured. It is well to take the reaction of 

 of potato. the potato with litmus before sterilisation, as this 

 varies ; normally in young potatoes it is weakly acid. 

 The reaction of the potato may be more accurately estimated by 

 steaming the potato slices for a quarter of an hour in a known 

 quantity of distilled water, and then estimating the reaction of 

 the water by phenol-phthaleine. The required degree of acidity 

 or alkalinity is obtained by adding the necessary quantity of 

 HC1 or NaOH solution (p. 35), and steaming for other fifteen 

 minutes. The water is then poured off and sterilisation con- 

 tinued for another half-hour. Potatoes before being inoculated 

 ought always to be incubated at 37 C. for a night, to make 

 sure that their sterilisation has been successful. 



Milk as a Culture Medium. 



This is a convenient medium for observing the effects of 

 bacterial growth in changing the reaction, in coagulating the 

 soluble albumin, and in fermenting the lactose. It is prepared 

 as follows : Fresh milk is taken, preferably after having had the 

 cream " separated " by centrifugalisation, as is practised in the 

 best dairies, and is steamed for fifteen minutes in the Koch ; it 



