54 



ESSENTIALS OF BACTERIOLOGY 



of the meat- water; or to the bouillon made from beef-extract 



the gelatin is added; this is placed in a flask and gently heated 



until the gelatin is dissolved. 



It is neutralized with the soda and then cooked in water-bath 



or carefully boiled over flame for one hour or more until the 



liquid seems clear, then add white of an egg and boil one- 

 quarter of an hour longer; 

 the egg will produce a clearer 

 solution and save much 

 trouble. A small portion, 

 while hot, is now filtered 

 into a test-tube and tested 

 for alkalinity, and then re- 

 heated several times, watch- 

 ing if a cloudy precipitate 

 forms. 



If the fluid remains clear 

 upon cooling, the remainder 

 of the material can be fil- 

 tered. It must be accom- 

 plished while hot, else the 

 gelatin will coagulate and 

 prevent further filtration. 

 This can be carried on 

 either by keeping hot the so- 

 lution continually in water- 

 bath, and only filtering a 

 small quantity at a time 

 through the filter, or keep- 

 ing the filter itself hot, either 



with a hot-water filter or placing the filter in steam-chest 



( Fi g- 23). 

 Clouding of Gelatin. If the gelatin does not come out clear 



or becomes turbid on cooling, it may be due to several things 



1. The filter-paper too thin or impure. 



2. Too strongly alkaline. 



3. Cooked too long or not long enough. 



Fig. 23. Hot-water filter. 



