44 METHODS OF CULTIVATION OF BACTERIA. 



as in i (a), but after filtration 6 to 8 per cent of glycerine (sp. 

 grav. 1.25) is added. This medium is especially used for 

 growing the tubercle bacillus when the soluble products of 

 the growth of the latter are required. 



2. Gelatine Media. These are simply the above broths, 

 with gelatine added as a solidifying body. 



2 (a). Peptone Gelatine. Take of meat extract say 

 1000 c.c., add 5 grams sodium chloride, 10 grams peptone, 

 and from 100 to 150 grams gelatine (the "gold label" gela- 

 tine of Coignet et Cie., Paris, is the best). The gelatine is 

 cut into small pieces, and added with the other constituents 

 to the extract. They are then thoroughly melted on a sand 

 bath, or in the " Koch," and the fluid neutralised as in i (a), 

 and filtered through filter paper. As the medium must not 

 be allowed to solidify during the process, it must be kept 

 warm. This is effected by putting the flask and funnel 

 into a tall Koch's steriliser, in which case the funnel must 

 be supported on a tripod, as there is great danger of the 

 neck of the flask breaking if it has to support the funnel 

 and its contents. The filtration may also be carried out in 

 a hot-water funnel (Fig. 7). This 

 consists of an outer tin funnel, the 

 neck of which is fitted with a 

 perforated cork, through which is 

 placed the stem of an inner glass 

 funnel, the diameter of whose 

 mouth is less than that of the outer 

 funnel. The interspace between 

 the two funnels is filled with water, 

 which is kept hot by a Bunsen 

 under a side arm let into the outer 

 funnel. 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 what first comes through is turbid. 

 If so, replace it in the un filtered part : often the subsequent 

 filtrate in such circumstances is quite clear. A litre flask 



FIG. 7. Hot-water funnel. 



