26 THE BACTERIOPHAGE AND ITS BEHAVIOR 



As an indication of penetrability, a sac prepared from collodion of 

 this composition will allow 55 cc. of water per hour per square decimeter 

 to pass through when under a pressure of 50 cm. of water (Duclaux). 



A very dense membrane, impermeable for ultraviruses of all kinds, 

 is obtained by preparing a collodion according to the following: 



Alcohol, 96 per cent 250 cc. 



Ether, 65 per cent 750 cc. 



Nitrate of cotton 50 



With a membrane prepared from this collodion a square decimeter 

 filters only 5 cc. of water per hour under a pressure of 1 meter of water 

 (Duclaux) . 



By varying the relative proportions of the constituents it is possible 

 to have in reserve a series of collodions suitable for preparing mem- 

 branes appropriate, as to porosity, to performing an experiment of 

 any type, whatever its nature. 



Preparation of ultrafiUers 



For the purpose of conducting investigations which must be carried 

 out aseptically, as is always the case in microbiological work, the sole 



* The following may be stated as practical suggestions. In order to obtain a 

 quick and complete dissolution of the cotton it is necessary to insure the absorp- 

 tion of the alcohol by first saturating the cotton with a small portion of the 

 alcohol, then add the ether little by little and when the mass has become trans- 

 parent, add the rest of the alcohol. Another rather important point is that 

 membranes prepared from a freshly made collodion are less homogeneous, and 

 less tough, than are those made from a "ripened" collodion. It is wise, there- 

 fore, to pour the freshly made up collodion into a tightly stoppered flask and to 

 keep it for a week in the incubator at 37°C. Once ripened the collodion may be 

 used indefinitely, provided it is properly preserved, at a low temperature by 

 preference and in tightly stoppered containers. I have not found these recom- 

 mendations definitely stated anywhere, yet they are "tricks" well-known in 

 some laboratories and of considerable importance. 



While on the subject of "tricks" I may mention one other. It is sometimes 

 very difficult to obtain membranes free of air bubbles, yet these can very readily 

 be avoided by placing the flask of collodion, still stoppered, in the incubator at 

 37° for a few hours before it is to be used. When the mold which is dipped into 

 the collodion to form the membrane has a temperature higher than that of the 

 liquid (and this is usually the case, because it is necessary to carefully dry the 

 mold with a soft cloth before immersing it in the collodion, and during this pro- 

 cedure the temperature of the hands heats the mold), the ether evaporating from 

 contact with the walls forms bubbles. This does not occur if the temperature of 

 the collodion is higher than that of the mold. 



