CULTURAL TECHNIQUE 5 



the standard solution to accurate work in chemistry. It is 

 desirable that the media shall be uniform in composition and 

 physical properties. This is, however, not wholly possible 

 with the present methods. Much greater uniformity than 

 usual might be attained, however, if standard ingredients 

 and uniform methods were insisted on. 



Ingredients of media. The following ingredients comprise 

 those used in the preparation of the ordinary media : 



Distilled water rather than tap water. 



Infusion of meat, Liebig's meat extract is often used 

 as a substitute on account of its greater convenience and 

 cheapness. 



Peptone, Witte's dry peptone prepared from meat. 



Gelatin, best French (gold label) brand. It is impossible 

 to obtain gelatin that has a constant composition. It should 

 be as free as possible from acids, and a 10 per cent solution, 

 when prepared as directed, should not soften at 25 C. 



Agar, commercial agar in threads of as high a grade as 

 can be obtained. Agar is prepared from certain seaweeds of 

 Ceylon and Japan, and is to be found on the market in the 

 form of threads, square sticks, and as a powder. 



Sugars, saccharose, lactose, and anhydrous dextrose, 

 used for the modification of standard media, should be as 

 nearly as possible chemically pure. The two latter should 

 be used in a powdered form. , 



Litmus, the ordinary litmus cubes contain but a small 

 amount of the pigment desired, azolitmin. It is preferable 

 to purchase a purified form of litmus, which should be free 

 from the reddish colorifics always found in the cubes, and 

 which interfere with the sensitiveness of the indicator. 



Reaction of media. The reaction of media is expressed 

 with reference to the neutral point of phenolphthalein, and 



