100 COLLOIDS IN BIOLOGY AND MEDICINE 



capillary completely immersed in and wet with water, a certain 

 pressure is necessary, which depends upon the surface tension of 

 water against air (which is a constant) and the diameter of the 

 capillary. If D is the diameter of the capillary, p the pressure in 

 atmospheres and |8 the capillarity constant, the following formula 

 applies : 



p. 1.033 + 10 5 



If j8 = 7.7 at 18, we obtain 



= p- 1.033 + 10 5 ' 



With the aid of this formula, the smallest diameter of the pores 

 in question may be calculated from the least pressure necessary to 

 drive air through the pores of the completely wet filter. 



The practical performance of the experiment is as follows: The 

 filtering apparatus is turned upside down, a thin layer of water is 



placed on the filter (several milli- 

 meters high) and the highest 

 pressure at which air bubbles 

 begin to escape is determined. 



C= Casing \ The diagrammatic sketch, Fig. 



F 19 ^' s ^ ows ^ ne filtering apparatus 



in normal position (T = funnel, 



F = ultrafilter, A = air intake). Fig. 19 shows it in the position 

 necessary for the forcing through of air. Above the filter there is 

 a thin layer of water. 



According to this method the largest pores of a filter which just 

 permits hemoglobin to pass through have a diameter 50 to 99 w- 



3. Method Based upon the Rate of Transfusion for Water. This 

 method affords approximately absolute values for the average diam- 

 eter of the pores of ultrafilters. The method is based upon the 

 somewhat indefinite law of Poiseuille for the passage of fluids through 

 capillary tubes. 1 

 D = diameter of the pores. 



Q = amount of water flowing through the surface F under the 

 constant pressure S. 



R is the ratio between the empty (water containing) space, and 

 that filled with solid. This is determined from the percentage of dry 

 material in the jellies (a 5 per cent filter contains full and empty 

 space in the proportion of 5 to 95). 



1 Bechhold.* 6 



