74 



SOLID CULTIVA TION MEDIA. 



acid, it must be neutralised by the addition of a saturated solution of 

 carbonate or alkaline phosphate of soda. This must be done very 

 accurately, and is best accomplished by using pieces of red and blue 

 litmus paper, adding the alkali in small quantities to the gelatine broth, 

 until an exactly similar tint is produced in each test paper on its 

 immersion in the liquid. As soon as its contents are completely 

 neutral in reaction, the flask is stoppered with a plug of sterile cotton 

 wool, and is placed in the steam steriliser already described (§ 39, p. 57), 

 which has been previously raised to a temperature of ioo° C. Here 

 the flask and its contents are steamed for thirty minutes, after which 

 they are allowed to stand for twenty-four hours in a cold place. On the 

 third day of the preparatory process the flask is again steamed for thirty 

 minutes, and while still liquid the contents are once more tested, and 

 more of the soda solution added, if they prove acid. If already 

 slightly alkaline, the broth may be left without further treatment, as 

 slight alkalinity of the soil is not unfavourable to the growth of many 

 species of micro-organisms. Whilst still liquid, the peptonised broth 

 should be filtered through coarse filter paper in a heated funnel 



Fig. 32. — Hot water funnel for filtering meat jelly. 



(see Fig. 32). This consists of a glass funnel, the neck of which fits 

 into an indiarubber plug, and around which is a copper funnel of 



