INTRODUCTION 29 



with a large capacity such as may be needed for special studies requir- 

 ing a large amount of filtrate, the dipping method is not suitable. 

 It is then necessary to proceed in the following manner. 



Prepare a suitable glass mold as has been indicated above, and attach 

 this mold, horizontally, to a shaft connected with a small motor. The 

 centering should be as perfect as possible and the speed of rotation 

 should be from one to two turns per second, according to the size of 

 the mold. With the mold rotating, pour a fine stream of collodion 

 upon the mold, beginning with the bulbous enlargement (the open 

 end of the sac) and proceeding toward the closed end of the sac. Per- 

 mit it to dry for an appropriate time, pour on some more collodion to 

 give a second layer, repeat a third time, and even a fourth, thus in- 

 creasing the toughness of the sac, which, being large, is proportionally 

 fragile. Remove the sac as with the smaller sacs. The larger the 

 sac, the more difficult is its construction. 



Assembling the ultrafilter 



Select a glass tube exactly* the same diameter as the exterior of the 

 mold, and some 20 to 25 cm. (or more in certain cases) in length. Long 

 tubes facihtate filtration, for in filling this supporting tube with the 

 fluid to be filtered sufficient pressure is obtained on the membrane to 

 effect filtration quickly, at least this is the case when using membranes 

 sufficiently porous to allow the passage of ultraviruses. 



Draw the mouth of the sac upon the supporting tube, to a height of 

 from 1.5 to 2 cm. Tie the sac to the tube with a fine and strong 

 string, introducing between the sac and the Hgature a band of parch- 

 ment paper to avoid tearing the sac (fig. 2-B). 



At no time during the manipulation should the sac be allowed to 

 dry, for with drying nitrocellulose membranes become impermeable, 

 even if they are again immediately moistened. It is, however, rela- 

 tively easy to keep them moist with water. 



With the sac thus attached to its supporting tube, take a tube of large 

 diameter and hah fill it with distilled water. Also fill the adjusted sac 

 with distilled water in such a way that the level of the water is some 

 4 to 5 cm. above the hgature. Place the sac within the large tube and 

 suspend it there at the desired height (the level of the water being the 

 same in both tubes) by means of a strip of cotton rolled tightly about 

 the supporting tube in such a way as to form a plug for the large tube. 



* It is well to measure the diameter exactly, with calipers. 



