SURFACES 17 



it should be possible to obtain a clear or a cloudy filtrate from an oil 

 emulsion according to the pressure employed. This assumption was 

 confirmed by an experiment of H. BECHHOLD: When an emulsion of 

 oil in water was filtered through a 3 per cent ultrafilter with a pres- 

 sure of 6 atmospheres a clear filtrate was obtained, and when the 

 pressure was increased to 10 atmospheres the filtrate became 

 turbid; with a decrease in the pressure the filtrate became clear 

 again. 



In my opinion the greatest importance of the method lies in the 

 ability to measure the surface tension of small fluid or semifluid 

 structures (e.g., blood corpuscles) and to deduce from such determi- 

 nations entirely new points of view concerning the passage of fluid 

 or semifluid structures through membranes. 



Let me point out another idea which forces itself upon me : namely, 

 that the sphere is the form most readily induced by surface tension. 

 If a solid substance separates from a fluid as a crystal we must 

 recognize that certain forces tending to increase the surface are 

 opposing the surface tension. But we know from the microscopic 

 study of crystal formation that spherical structures usually appear 

 first; later, crystalline forms with rounded corners, and only in the 

 later stages true crystals. 1 There must be a certain relation between 

 mass and surface in order that the solid phase may be elevated 

 above the surfaces bounded by planes in defiance of the surface 

 tension. If the surface is too great in proportion to the mass, the 

 surface tension overcomes the crystallizing forces. Since it is possi- 

 ble to estimate the increase in surface acquired by the identical 

 substance in changing from a spherical form to a crystal, and further, 

 to observe the smallest quantity of a substance which can become 

 crystalline, it becomes possible to solve many problems, such as, the 

 effective forces of crystallization, the surface tension which solid bodies 

 exert against their solutions, and the decreased capacity to crystal- 

 lize in the presence of colloids. 



A drop of oil spreads out over the surface of water. This occurs be- 

 cause the surface tension of water acting against oil is less than the 

 surface tension of the water acting against air minus the surface 

 tension of the oil acting against air. (The explanation of this is 

 given in all the larger text books on Physics.) 



o- water/air > <r water/oil + o- oil/air 

 75 > 22.9 + 32.7. 



1 Bibliography in Wo. OSTWALD, " Handbook of Colloid Chemistry," trans, by 

 Fischer, Oesper and Berman, Phila., 1915. 



