34 THE BACTERIOLOGICAL EXAMINATION OF MILK 



as above, which is now returned to the bulk, will counteract this. 

 Proceed now more cautiously, and repeat the test, adding the 

 soda solution to the bulk drop by drop from a pipette, until the 

 first tinge of pink is seen against the white porcelain after stirring 

 the mixture with a glass rod. The medium is now neutral to 

 phenolphthalein. Keep it at the boiling-point for another five 

 minutes, and then, proceed as laid down under preparation of 

 media. 



Care must be taken to thoroughly cleanse the spoon employed 

 for the medium by dipping in hot water between each of the 

 above operations, and the medium itself must be kept constantly 

 and well stirred by an assistant. 



Peptones 



Very little reliance can be placed in the uniformity of the 

 dry peptones of commerce. Even those of the best makers vary 

 at times in a high degree, and all quickly lose their nutritive 

 properties with age. The authors have found the liquid peptone 

 {bouillon d'estomac de pore) originally suggested by Dr Louis 

 Martin for the production of diphtheria toxin so easy in manipu- 

 lation, economical in use, and uniform in result, that they have 

 for some years past, in the preparation of media, used it to the 

 exclusion of all others. Its preparation is as follows, and as it 

 improves somewhat with age a sufficient stock can be made at 

 one time and kept in hand for use. 



Preparation. — Take two or three stomachs of freshly killed 

 pigs, and mince together i kilogramme of the mucous and 

 muscular coats. Place in a strong earthenware jar containing 

 5 litres of water acidulated with 50 c.c. (i per cent.) pure hydrochloric 

 acid, and heat up to 50°. Allow to digest for twenty-four hours, 

 maintaining the temperature at 50°, at which the pepsin of the 

 mucous lining digests the tissues more actively and transforms 

 them into peptone. When digestion has taken place the bouillon 

 is brought slowly to the boiling point, by heating the jar over the 

 Bunsen flame, in order to destroy the excess of pepsin. The liquid 

 obtained by this self-digestion of the stomach is a true solution of 

 peptone. It is then filtered through a fine sieve, or a layer of 

 loosely packed cotton wool, and set aside to cool. In this condition 

 it can be stored for apparently an unlimited time without deteriora- 

 tion. A slight layer of mould will form on the surface of the jar, 

 but this can be disregarded, and the strong acidity prevents the 



