152 BIOLOGY AND TECHNIQUE 



The medium is then titrated and adjusted. After cooling, the whites of 

 two eggs are added and the media is put in the Arnold for forty-five minutes. 

 The medium is filtered, tubed and sterilized by the fractional method. All 

 unnecessary heating of gelatin is to be avoided. 



Meat Infusion Gelatin. Meat infusion gelatin is made in the same way 

 as the above except that fresh meat infusion is substituted for the meat 

 extract. 



Meat-Extract Agar. To 1 liter of distilled or tap water add: 



Thread agar 15 to 30.0 grams 15 



Pepton 10.0 ' ' 



. . Meat extract . 5.0 " 



Common salt 5.0 " 



Put in autoclave, 15 pounds' pressure, for 15 minutes. The agar can 

 also be dissolved over the free flame, but this takes a long time and is not 

 necessary. However, if so done make up loss by evaporation. 



Take out of autoclave, and adjust to desired reaction. 



Cool to 60 C. and add the whites of two eggs. Stir thoroughly. 



Heat either in the autoclave, fifteen pounds, thirty minutes; or if no 

 autoclave is available, thirty minutes in the Arnold. If the heating is done 

 in the Arnold, take out after half an hour, stir and replace for fifteen 

 minutes more. 



Titrate, and adjust again if reaction has changed. 



If a correction in reaction is made, heat again for ten minutes, filter 

 through cotton, tube and sterilize. 



Meat Infusion Agar. 16 In making meat infusion agar it is best to prepare 

 two solutions, one consisting of the meat infusion to which the pepton and 

 salt are added, the other an aqueous solution of agar which is dissolved 

 in the autoclave. The ingredients are added in double strength to each 

 solution so that when they are finally combined, the desired concentrations 

 are obtained. 



Solution (1) 500 grams of lean meat in 500 c.c. of distilled or tap water 

 are infused over night in the ice-box. The infusion is then strained through 

 cheese cloth and all the juice squeezed out of the meat. The infusion is 

 measured and the volume made up to 500 c.c. Two per cent pepton or 

 10 grams, and 1 per cent or 5 grams salt are now added to the 500 c.c. 

 infusion and the mixture is heated over ihe free flame until it reaches a 

 temperature of about 50 C. and the pepton dissolves. This solution is then 



15 Amount of agar varies according to stiffness desired. 



16 While titrating care should be taken that the medium does not solidify along 

 the sides of the vessel. Glycerin agar is made by adding 6 per cent C.P. glycerin 

 to meat extract or meat infusion agar. 



