PEPTONE GELATINE. 



47 



FIG. 7. Hot-water funnel. 



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 inter- 

 space 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 unfiltered 

 part : often the subsequent filtrate 

 in such circumstances is quite 

 clear. A litre flask 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 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. 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 on three successive days. 

 Too much boiling, or boiling at too high a temperature, as 

 has been said, causes a gelatine medium to lose its property 

 of solidification. 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, 15 parts per 100 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 



