80 BACTERIOLOGY. 



cess of filtration much more difficult, because of the 

 disintegration of these masses into the finer particles, 

 which have the effect just -mentioned. 



Under no circumstances is a filter to be used 

 without first having been moistened with water. If 

 this is not done, the pores of the paper, which are 

 relatively large when in a dry state, when moistened by 

 the gelatin not only diminish in size, but in contract- 

 ing are often entirely occluded by the finer albuminous 

 flakes which become fixed within them, and filtration 

 practically ceases. The preliminary moistening with 

 water causes the diminution of the size of the pores 

 to such an extent that the finer particles of the precipi- 

 tate rest on the surface of the paper, instead of becoming 

 fixed in its meshes. 



During boiling it is well to filter, from time to time, 

 a few cubic centimetres of the gelatin into a test-tube 

 and boil it over a free flame for a minute or so ; in this 

 way one can detect if all the albumin has been coagu- 

 lated and when the solution is ready for filtration. 



Gelatin should not, as a rule, be boiled over ten or 

 fifteen minutes at one time, or left in the steam sterilizer 

 for more than thirty to forty-five minutes, otherwise its 

 property of solidifying is materially diminished. 



As soon as the gelatin is complete, whether it is 

 retained in the flask into which it has been filtered or 

 decanted off into sterilized test-tubes, it should be steril- 

 ized in the steam sterilizer on three successive days, for 

 fifteen minutes each day — the mouth of the flask or the 

 test-tubes containing it having been previously closed 

 with cotton plugs. 



Nutrient Agae-a.gae. — The preparation of nu- 

 trient agar-agar by the beginner is, as a rule, a some- 



