50 



ESSENTIALS OF BACTERIOLOGY. 



FIG. 23. 



This is placed in a flask and gently heated until the gelatine 



is dissolved. 



Neutralization with the soda and then cooking in water-bath 



for 1 hour or more until the liquid seems clear, then add white 



of an egg and boil \ hour 

 longer ; the egg will produce 

 a clearer solution and save 

 much trouble. A small por- 

 tion, while hot, is now filtered 

 into a test-tube and tested for 

 alkalinity, and then re-heated 

 several times, watching if a 

 cloudy ppt. forms. 



If the fluid remains clear 

 upon cooling, the remainder of 

 the material can be filtered. 

 It must be accomplished 

 while hot, else the gelatine 

 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 keeping the filter 

 itself hot, either with a hot 

 water filter or placing the 

 filter in steam chest. (Fig. 

 23.) 

 Clouding of Gelatine. If the gelatine 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. 



The addition of the white of an egg, as before mentioned, will 

 often clear it up ; if this avails not. re-filtering several times, and 

 attention to the few points mentioned. 



Hot-water filter. 



