GELATINE AND AGAR MEDIA 45 



of the finished product ought to be quite transparent. If 

 instead it is partially opaque, add the white of an egg and 

 boil thoroughly over the sand bath. The consequent coagu- 

 lation 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. 37. If the autoclave be 

 used the temperature employed must not be above 105 C., 

 and exposure not more than a quarter of an hour. Too much 

 boiling, or boiling at too high a temperature, as has been 

 said, causes a gelatine medium to lose its property of solidi- 

 fication. This transparent solid gelatine medium is that 

 chiefly employed for the culture of aerobic bacteria at 

 ordinary temperatures. The exact percentage of gelatine 

 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 summer 

 weather, 1 5 parts per i oo are necessary. A limit is placed 

 on higher percentages by the fact that, if the gelatine be too 

 stiff, it will split on the perforation of its substance by the 

 platinum needle used in inoculating it with a bacterial 

 growth ; 15 per cent gelatine melts at about 24 C. 



2 (b). Glucose Gelatine. The constituents are the same 

 as 2 (), with the addition of i 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, "gelose"). The disadvantage 

 of gelatine is that at the blood temperature (38 C.), at 

 which most pathogenic organisms grow best, it is liquid. 

 To get a medium which shall be solid at this temperature, 

 agar is used as the stiffening agent instead of gelatine. 

 Unlike the latter, which is a proteid, agar is a carbohydrate. 

 It is derived from the stems of various sea-weeds growing 

 in the Chinese seas, popularly classed together as " Ceylon 

 Moss." The best for bacteriological purposes is that 

 consisting of the thin dried stem of the sea-weed itself. 



