28 THE BACTERIOPHAGE AND ITS BEHAVIOR 



Immediately after removal from the collodion the mold should be 

 held horizontally and should be rotated continually until the gel 

 attains the requisite consistency. If the evaporation has not been 

 sufficient the sac is hkely to tear during removal from the mold; if 

 drying has proceeded too far, the sac is liable to wrinkle and is removed 

 from the mold with difficulty. With a Httle experience one is able to 

 sense the opportune moment; if one employs the time intervals indi- 

 cated in the preceding note a membrane of the requisite consistency is 

 obtained after about a minute and a half. At this moment, plunge 

 the mold into the collodion a second time, observing the same pre- 

 cautions as in the first immersion, and withdrawing at the same speed. 

 Allow it to dry somewhat, holding it horizontally while continuously 

 rotating it between the fingers. With the second layer permit the 

 evaporation to continue for a sfightly longer time, about two minutes.* 



When the membrane thus prepared is of the requisite consistency, 

 plunge the mold into water for a few seconds, then, separating with the 

 finger nail the upper margin of the sac from the middle portion of the 

 bulb, under a stream of running water, take hold of the loosened 

 portion and turn it back on itself fike the finger of a glove. The ex- 

 terior surface of the membrane when the sac was upon the mold thus be- 

 comes, after removal, the interior. Place the sac in distilled water, 

 or, if it is not to be assembled for use at once and if it is desired to pre- 

 pare a supply of sacs ready for use, they should be immersed in an 

 aqueous 20 per cent alcohol. f 



The reader must not assume from this detailed description that the 

 preparation of ultrafilters is difficult. After a few minutes' practice 

 one "gets the knack" if one has the least manual dexterity. 



For sacs with a diameter greater than 4 or 5 cm., that is, for sacs 



interest. For the more fluid collodions the removal of the mold should require 

 10 to 15 seconds, for the more dense collodions this period should be extended to 

 30 to 45 seconds. 



* Naturally the period of drying varies with the surrounding temperature, the 

 presence or absence of air currents, etc. , since anything which hastens evaporation 

 shortens the time. In any case, the first sac made provides an index for further 

 procedure. If the sac tears in the process of removal the period of drying must 

 be extended; if it is wrinkled, stiff, and is detached with difficulty, drying has gone 

 too far. 



t If the proposed investigation is to demand a great many filtrations, it is well 

 to prepare at one time enough sacs to meet all needs. In the first place this will 

 save time, and in the second place, and this is of particular importance, it will 

 tend to give greater uniformity in the series of sacs to be employed in the study. 



