98 



COLLOIDS IN BIOLOGY AND MEDICINE 



the cover D with a conical joint and a rubber washer. By turn- 

 ing the screw cap Schr, the cover above as well as the filter below 

 is tightened. A small nipple with a screw thread passes through the 

 cover and to this the pipe from the pressure chamber is attached. 



The apparatus shown in Fig. 17 is chiefly 

 used for pressures above 10 atmospheres, 

 and is closed with flanges. Naturally 

 this is more bulky. The lettering in 

 Fig. 17 corresponds with that of Fig. 16, 

 so that it is unnecessary to duplicate the 

 description. An apparatus with a stirrer 

 (also on the market) is usually prefer- 

 able, because filtration is relatively more 

 rapid and the filtrate is more uniform. 

 The omission of stirring may permit a 

 gel layer to form on the ultrafilter, and 



tws gei iayer may then act as a mter 



itself. It is especially important to have 

 the packing tight against high pressures. In this apparatus the 

 pressure is introduced through a side opening because the stirrer 

 occupies the central one. 



The Pressure. The pressure may be produced by a hand 

 pump. This is especially useful in the scientific investigation of the 

 action of filters, where fine gradations of pressure are involved, and 

 where prolonged pressure is unnecessary. In practical ultrafiltra- 

 tion,' it is preferable to use a steel cylinder containing either com- 

 pressed air, nitrogen or carbonic acid, etc. Between the steel 

 cylinder of the ultrafiltration apparatus, a reducing pressure valve and 

 two manometers are introduced; one for very high pressure shows 

 the pressure in the cylinder, the other, beyond the valve, the lower 

 pressure in the ultrafiltration apparatus. A second reducing pressure 

 valve near this manometer permits such delicate differences in 

 pressure that, in my opinion, it is safe to use this arrangement in- 

 stead of the hand air pump even in scientific measurements. 



The extensive use achieved by BECHHOLD'S ultrafiltration has led 

 to some modifications for special purposes. ZSIGMONDY, WILKE- 

 DORFURT and GALECKI recommended collodion skins for analytical 

 purposes. They placed collodion skins on a BUCHNER funnel (Goocn 

 filter, i.e.) porcelain funnel with perforated diaphragm) and thus 

 filtered off coarse colloids, especially inorganic ones, by means of a 

 tap-water suction pump. This arrangement is not suitable for 

 pressures above one atmosphere. [J. F. MCCLENDON employs alun- 

 dun thimbles coated with collodion. Tr.] 



The essential new point in the apparatus devised by BURIAN E. 



