36 METHODS OF CULTIVATION OF BACTERIA 



while melting to have settled into layers of different density. 

 Sometimes what first comes through is turbid. If so, replace it 

 in the unfiltered part : often the subsequent filtrate in such cir- 

 cumstances 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 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. 27. 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 summer weather, 15 parts 

 per 100 are necessary. A limit is placed on higher percentages 

 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 ; 15 per cent gelatin melts 

 at about 24 C. 



2 (&). 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, "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 sea-weeds growing in the Chinese seas, popularly 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 ; this is preferable. 



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

 tion : 



Meat extract . . . . . . 1000 c.c. 



Sodium chloride . . . . * 5 grms. 



Peptone albumin . l . . . 10 ,, 



Agar 15 



