GELATIN MEDIA 



37 



also be carried out in a funnel with water-jacket which is heated, 

 as shown in Fig. 7. Whichever instrument be used, before 

 filtering shake up the melted medium, as it is apt while melting 

 to have settled into layers of different density. Sometimes 

 the first portion of filtrate is turbid. If so, replace it in the 

 unfiltered part : often the subsequent filtrate in such circum- 

 stances 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 steam 

 thoroughly. 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. 28. If the auto- 

 clave be used the temperature em- 

 ployed 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 pre- 

 paration" 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 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 ; 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 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 temjierature (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 



Fig. 7. — Hot-water funnel. 



