38 CULTIVATION OF BACTERIA. 



Some objection is raised to the use of sodium solutions as 

 described for alkalinizing the bouillon, as it is so easy to add 

 too much. 



The reaction may also be taken by Sehultze's method. He 

 uses a 0.33 per cent, alcoholic solution of phenolphthalein 

 and sodium hydroxide, which is added with a burette until 

 a faint red color appears. This indicates a faintly alkaline 

 reaction. 



The first method is certainly less complicated, more easily 

 performed, and when carefully carried out gives the same 

 results. The sodium bicarbonate must be added slowly, a 

 little at a time, and the mixture constantly stirred so that 

 the soda will be thoroughly distributed. 



The final product may be modified by the addition of from 

 1 to 3 per cent, of glucose, lactose, or saccharose. This is 

 known as sugar-bouillon. When it is desired to have a 

 medium free from sugar, the grape-sugar contained in all 

 meat-extracts is removed from the bouillon by inoculating it 

 with the colon bacillus, which destroys the sugar by fermenta- 

 tion. After twelve hours the bouillon is filtered and the 

 resulting clear fluid is free from sugar. This bouillon can 

 also be used for making accurate fermentation-tests, as the 

 correct percentage of sugar can be added. 



Nutrient gelatin : Gelatin has the same nutrient value as 

 beef-tea, but possesses the additional advantage of being solid. 

 It cannot, however, be placed in the incubator, as its melting- 

 point is 25 C. It is made from the beef-tea stock with the 

 addition of 10 per cent, of gelatin : 



Beef-extract, 2 grams ; 



Peptone, 10 " 



Salt, 5 " 



Gelatin (gold label), 100 " 



Water, 1000 c.c. 



The various ingredients are dissolved as in the case of the 

 bouillon. The gelatin is broken up finely and added. The 

 mixture is then boiled until the geJatin is completely dis- 

 solved. It is well to stir the solution constantly, as the gela- 



