34 



THE BACTERIOPHAGE AND ITS BEHAVIOR 



adjusting the sac to a longer tube) is adequate to accomplish the 

 ultrafiltration unless the membrane is too dense. If it is necessary to 

 use very dense ultrafilters (for example, when it is desired to retain 

 the ultra virus and thus to obtain an "ultrasterile" ultrafiltrate, as is 

 essential in carrying out studies upon the secretory products of the 

 ultra viruses) pressure must be apphed. For such purposes it is wise 

 to prepare sacs, not with two layers of collodion, but with 4 or 5, in- 

 creasing thus the toughness of the membrane. They should be deni- 

 trified, and assembled as has been outlined, except that in the place 



Fig. 5. The Ultrafilter Assembled as (A) an Ultrafilter and (B) a 

 DiALYZiNG Sac 



of the large tube into which they are suspended, they should be intro- 

 duced into a tube with a side neck. It is also obvious that it is then 

 necessary to adapt the tube bearing the sac to the side-necked tube 

 by means of a perforated rubber stopper. To the reader all of these 

 manipulations may appear compHcated, but as a matter of fact, a 

 little experience will show that the preparation of ultrafilters, and 

 ultrafiltration itself, are but simple procedures. 



