AGAR MEDIA 37 



tion of the albumin carries down the opalescent material, and, 

 on making up with distilled water to the original quantity and 

 refiltering, it will be found to be clear. The flask containing it 

 is then plugged with cotton wool and sterilised, best by method 

 B (2), p. 28. If the autoclave be used the temperature employed 

 must not be above 105 C., and exposure not more than a quarter 

 of an hour on three successive days. Too much boiling, or boil- 

 ing at too high a temperature, as has been said, causes a gelatin 

 medium to lose its property of solidification. The exact percentage 

 of gelatin used in its preparation depends on the temperature at 

 which growth is to take place. Its firmness is its most valuable 

 characteristic, and to maintain this in hot summer weather, 15 

 parts per 100 are necessary. A limit is placed on higher per- 

 centages by the fact that, if the gelatin be too stiff, it will split 

 on the perforation of its substance by the platinum needle used 

 in inoculating it with a bacterial growth ; 1 5 per cent, gelatin 

 melts at about 24 C. For ordinary use in British laboratories 

 10 per cent, gelatin is a sufficient strength. 



2 (6). Glucose Gelatin. The constituents and mode of pre- 

 paration are the same as 2 (a), with the addition of 1 to 2 per 

 cent, of grape sugar before sterilisation. This medium is used 

 for growing anaerobic organisms at the ordinary temperatures. 



3. Agar Media (French, "gelose"). The disadvantage of 

 gelatin is that at the blood temperature (38 C.), at which most 

 pathogenic organisms grow best, it is liquid. To get a medium 

 which will be solid at this temperature, agar is used. as the 

 stiffening agent instead of gelatin. Unlike the latter, which 

 is a proteid, agar is a carbohydrate. It is derived from the 

 stems of various seaweeds growing in the Chinese seas, com- 

 mercially classed together as " Ceylon Moss." For bacteriological 

 purposes the dried stems of the seaweed may be used, but there 

 is in the market a purified product in the form of a powder, 

 which is preferable. 



3 (a). "Ordinary" Agar. This has the following composi- 

 tion : 



Meat extract . . . . 1000 c.c. 



Sodium chloride .... 5 grms. 



Peptone albumin . . . . 10 ,, 



Agar . . . . . . 15 



Cut up the agar into very fine fragments (in fact till it is as 

 nearly as possible dust), add to the meat extract with the other 

 ingredients, and preferably allow to stand all night. Then boil 

 gently in a " Koch " for two or three hours, till the agar is 



