16 



COLLOIDS IN BIOLOGY AND MEDICINE 



ultrafilter. The author raised certain objections to these conclusions, 

 as it was not necessary to assume that the drops of oil retained their 

 form in passing through the filter pores; they might have been drawn 

 out into thread-like cylinders, and their diameter thereby reduced. 

 As a result of the correspondence that followed, H. BECHHOLD pro- 

 posed that the pressure necessary to change the shape of a sphere of 

 oil in a fluid be estimated and experimentally verified. 



E. HATSCHEK* carried out these calculations and measurements. 

 The method of calculation is in brief the following : Upon entering a 

 capillary, a sphere changes into a cylinder which is bounded above 

 and below by a meniscus; to simplify matters these meniscuses are 

 regarded as hemispheres. Accordingly, let 



then 



then, 



R = radius of the oil sphere, 

 r = radius of the capillary, 

 R = nr; 



<j = surface tension in dynes/cm., 

 p = pressure per surface unit of the emulsion, 

 g = 980 (acceleration due to gravity), 

 an 



n approximately = C (n 1), so that C lies between 1.8 and 



1.9, and we have the equation p = - ~ - - ; or, if we desire to de- 



Hg 



termine the size of the pores, n *-- + 1 ; or, should we desire to 



Go" 



determine the surface tension <r = , ., N 



C (n 1) 



E. HATSCHEK tested the correctness of the formula by determining 

 the pressure necessary to deform a sphere of mercury or oil (nitroben- 

 zol) sufficiently to make it enter a narrow capillary and obtained 

 satisfactory results. To give an idea of the pressures that are in- 

 volved, let us consider the following example: In order to force a 

 drop of mercury with a radius of 0.111 cm. in water into a capillary 

 with a radius r, a water column of p centimeters was necessary. 



In an emulsion of oil in water in which the diameter of the oil drops 

 was 0.4 /j, a pressure of 20 atmospheres was required to press them 

 through pores having a diameter of 75 m*. According to this theory 



