40 THE BACTERIOLOGICAL EXAMINATION OF MILK 



should on no account exceed 105°. This medium is specially useful 

 for showing the peptonising properties of certain organisms at 

 blood heat. If carefully made the medium remains solid up to 48°, 

 liquefaction, however, by peptonisation taking place as in the case 

 of gelatine alone, although in a lesser degree. A very valuable 

 medium for use in the tropics. 



19. Glucose formate agar — 



Add to hot melted agar during preparation, glucose and sodium 

 formate as under the head of " glucose formate gelatine." 



20. Nutrient milk agar — 



Thoroughly melt a tube of nutrient agar in a beaker of boiling 

 water. Place a tube containing 5 c.c. of sterile milk in the beaker 

 and heat up to between 70° and 80°, but do not allow it to approach 

 the boiling point. Remove the cotton-wool plugs and quickly pour 

 an equal quantity (5 c.c.) of the melted agar into the tube contain- 

 ing the milk. Replace the cotton-wool plug and allow to cool 

 either vertically or on the slope, as required. A nutrient milk 

 gelatine can be made in the same manner provided the proportion 

 of gelatine in the medium is not less than 1 5 per cent. 



21. Potato {in tubes) — 



Choose long, sound, and healthy potatoes of the same crop if 

 possible.^ Thoroughly scrub them with a hard brush in a current 



of water to remove all trace of 

 soil. Cut off both ends of the 

 potato with a clean knife, and 

 bore through the longer axis 

 with a cork borer ^ of f in. dia- 

 ¥\G. \.—?ota.to hor&r: Emporte-piece. meter (for tubes of | in. in- 



ternal diameter). Push out 

 the cylinder, and allow it to drop on to a clean filter paper. 

 Without touching it with the hands, and using a clean knife 

 previously boiled, cut it through on its longer axis into two 



^ The different varieties of potato vary greatly in composition and reaction, 

 and the difference in cultural properties of some of the varieties is very striking. 



^ If a special " emporte-piece " is procurable, it should be used, as with this 

 the cylinders are, in one and the same operation, bored and divided into equal 

 halves, and can be dropped directly into the soda solution without handling. 

 This, can now be procured from Messrs Baird & Tatlock of 14 Cross Street, 

 Hatton Garden. A nickled borer should in any case be employed, as the slight 

 traces of iron salts left on the potato when bare steel is used will often modify 

 considerably the colouring properties of chromogenic organisms. 



