AGAR MEDIA 37 



part : often the subsequent filtrate in such circumstances is 

 quite clear. A litre flask of the finished product ought to be 

 quite transparent. If, however, it is partially opaque, add 

 the white of an egg, shake up well, and boil thoroughly over 

 the sand bath. The consequent coagulation of the album in 

 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 boiling 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 (b). Glucose Gelatin. The constituents are the same as 

 2 (a), with the addition of 1 to 2 per cent, of grape sugar. The 

 method of preparation is identical. This medium is used for 

 growing anaerobic organisms at the ordinary temperatures. 



3. Agar Media (French, " ge'lose "). 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 : 



Mi-at extract 1000 c.c. 



Sodium chloride .... 5 grms 



Peptone albumin . . . . 10 

 ir 15 



