128 BACTERIOLOGY 



both gelatin and agar-agar may be rapidly filtered if they 

 are completely dissolved. 



It not infrequently occurs that, even under the most 

 'careful treatment, the filtered gelatin is not quite trans- 

 parent, and clarification becomes necessary. For this 

 purpose the mass must be redissolved, and when at a tem- 

 perature between 60 and 70 C. an egg, which has been 

 beaten up with about 50 c.c. of water, is added. The whole 

 is then thoroughly mixed together and again brought to the 

 boiling-point, and kept there until coagulation of the 

 albumin occurs. The egg albumin coagulates as large floccu- 

 lent masses, and it is better not to break them up, as when 

 broken up into fine flakes they clog the filter and materially 

 retard filtration. 



The practice sometimes recommended of removing these 

 albuminous coagula by first filtering the gelatin through a 

 cloth, and then through paper, is not only superfluous, but 

 in most instances renders the process of filtration much more 

 difficult, because of the disintegration of the masses into 

 finer particles, which have the effect just mentioned, viz., 

 of clogging the filter. 



Under no circumstances should a filter 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 contracting are often entirely occluded by the 

 finer albuminous flakes which become fixed within them, 

 and filtration practically ceases. The preliminary moisten- 

 ing with water causes diminution of the size of the pores to 

 such an extent that the finer particles of the precipitate rest 

 on the surface of the paper, instead of becoming fixed in its 

 meshes. 



