38 THE BACTERIOLOGICAL EXAMINATION OF MILK 



11. Carbolised gelatine — 



Dissolve in the liquefied gelatine prior to final filtration carbolic 

 acid crystals in the proportion of 5 grammes (-5 per cent), i 

 gramme (-i per cent), or 5 decigrammes (-05 per cent) per litre, 

 according to strength desired. 



12. Glucose formate gelatine (Kitasato) — 



To 500 c.c. of hot nutrient gelatine just prior to final filtration, 

 add 2 grammes (-4 per cent) of sodium formate, and 10 grammes 

 (2 per cent.) of anhydrous glucose. Dissolve and thoroughly mix 

 before filtering. 



1 3. Glucose {or lactose^ gelatine — 



As above, with omission of the sodium formate. 



1 4. Z itmus gelatine — 



Liquefy a tube or tubes of nutrient gelatine by placing in hot 

 water, and by means of a finely-drawn Pasteur pipette introduced 

 between the cotton wool plug and the tube wall, add sufficient 

 sterile litmus solution to tint the medium a deep lavender colour. 

 Roll well in the hand to ensure an even distribution of the colour 

 and allow to cool either upright or on the slope as required. 



Preparation of neutral litmus solution. — Extract all the colouring-matter 

 from solid litmus by repeated digestion with hot water. Evaporate this solution 

 to a moderate volume, and convert all carbonates present into acetates by add- 

 ing a slight excess of acetic acid. Again evaporate the solution over a water- 

 bath until it becomes pasty, and then add an excess of alcohol. The spirit will 

 precipitate the blue colouring-matter, while a red colouring-matter, together 

 with the alkaline acetates, will remain in solution. The precipitate is trans- 

 ferred to a filter, and is washed with spirit. It is then dissolved in warm water, 

 and the solution is rendered purple by the cautious addition of dilute nitric 

 acid. 



15. Nutrient agar {to make i Htrc^ — 



Proceed as in the case of nutrient gelatine up to and including 

 the addition of the salt Bring up to the boiling point and 

 neutralise to phenolphthalein in the manner described under 

 neutralisation of media. Boil for five minutes. Filter hot and 

 make up to i litre with distilled water. Add, after compressing 

 in a cloth to get rid of excess of moisture, 20 grammes of agar 

 cut into small pieces and previously soaked in distilled water for 

 eighteen hours. Steam in water bath until agar is thoroughly 

 dissolved, stirring constantly. Cool down to 60°. Add the white 

 of one Q.^^ in 60 c.c. of distilled water, and thoroughly incor- 



