NUTRIENT MEDIA. 49 



other media, must be carefully neutralized with a sat. sol. of 

 carbonate of soda. Since too much alkalinity is nearly as bad 

 as none at all, the soda must be added drop by drop until red 

 litmus paper commences to turn blue. The bouillon is then 

 cooked another hour, and filtered when cold. The liquid thus 

 obtained must be clearly alkaline, and not clouded by further 

 cooking. If cloudiness occur, the white of an egg and further 

 boiling will clear the same. To make bouillon, beef-extract can 

 be used instead of fresh meat, 2 grammes to 1 litre of water. 

 This is boiled with 5 grammes of salt and 10 of peptone, neu- 

 tralized as above, and filtered when cold. 



Schult^s Method of Neutralization. A more accurate method of 

 obtaining the required reaction is to use an alcoholic solution 

 ( per cent.) of phenolphthalein as an indicator ; a few drops of 

 this are mixed with 10 c.c. of the bouillon, and from a burette a 

 solution of caustic soda 0.4 per cent, is added drop by drop 

 until a faint red color appears. An average is taken from three 

 different samples, and the amount of soda needed for the entire 

 quantity of bouillon is calculated therefrom. Glucose broth, 

 which is a good medium for anaerobic organisms, consists of 

 bouillon to which 1 to 2 per cent, of grape-sugar has been 

 added. Glycerin broth is bouillon to which 6 to 8 per cent, of 

 glycerin has been added after filtration. 



Sterilization of the Bouillon. Erlenmeyer flasks (little conical 

 glass bottles) or test-tubes plugged and properly sterilized are 

 filled one-third full with the bouillon, and placed with their 

 contents in the steam-chest. They are left in steam of 100 C. 

 one hour for three successive days, after which the tubes and 

 bouillon are ready for use. 



Solid Media, The knowledge of bacteria and germs or moulds 

 settling and growing upon slices of potato exposed to the air, led 

 to the use of solid media for the artificial culture of the same. 

 It was also thus learned that each germ tends to form a separate 

 colony and remain isolated. 



Potato-Cultures. A ripe potato with a smooth skin is .the 

 best. 



Several are brushed and scrubbed with water to get rid of the 

 dirt and the "eyes" are cut out. 

 4 



