INTRODUCTION 



27 



practical method of assembling the ultrafilter is that which was first 

 described by Malfitano. The ultrafilter, which has the form of a 

 sac, is adjusted to the end of a glass tube of the same diameter. 



It is first necessary to prepare a mold. Unless one has in view some 

 special investigation necessitating a large quantity of ultrafiltrate 

 (and then it is necessary to select a mold of appropriate size) filter 

 sacs with a diameter of 15 mm. and a height of from 6 to 8 cm. are 

 to be preferred. To make the mold a test-tube of the appropriate size 

 is selected and by means of a blow-pipe a bulbous enlargement is 





A B 



Fig. 2. A, Mold for Preparing the Ultrafilter; B, the Celloidin Sac 



Adjusted to Supporting Tube 



a, glass supporting tube; b, ligature; c, celloidin sac 



formed near the middle of the tube (fig. 2). The mold being perfectly 

 clean and thoroughly dry, and at a temperature slightly lower than that 

 of the collodion (see the note on the preceding page), is plunged ver- 

 tically into the collodion up to the point where the surface of the liquid 

 reaches the middle of the bulb. It is then withdrawn very slowly and 

 at a uniform rate.* 



* It is well to have a watch available so that the same length of time can always 

 be used for the different procedures, thus permitting the sacs to be comparable as 

 regards porosity. The greater the viscosity of the collodion the more slowly 

 should the mold be removed, otherwise membranes are obtained which are too 

 thick. As more specifically indicating the procedure the following may be of 



