I. FUNDAAEENTALS OF SUBAETCROSCOPIC 



MORPHOLOGY 



"Le cytoplasnie propremeiit dit se present e sur le vivant 

 comme une substance collo'idale homogene, translucide, op- 

 tiquement vide a I'ultramicroscope. . ." 



GuiLLiF.RMOND, Mangf.not et Plantefol 



(1933, p. 386) 



§ I. Organization of Sols 



a. Invisible Particles 



Ever since Graham (1861) showed that the pseudo-solutions which 

 impede filtration and which nowadays we call sols contain relatively 

 large, slowly diffusing particles, the nature of these invisible particles 

 has been explored in all directions by colloid chemistry (Zsigmondy, 

 1925; OsTWALD, 1927). 



Demonstration and shape of the particles. Numerous methods have been 

 worked out to distinguish and to separate the originally hypothetical 

 submicroscopic colloid particles from the amicroscopic molecules. 

 By means of dialysis the amicroscopic particles can be made to 

 permeate through a semi-permeable membrane (parchment) through 

 which the colloid particles cannot follow (Graham, 1862). This 

 method has since been developed into ultrafiltration^ by which sols are 

 pressed through filters with submicroscopic pores (collodion films of 

 varying pore size) and in this way are split up into fractions of different 

 particle sizes. Further, since most colloid pa'rticles carry an electric 

 charge or can be charged by a change in the acidity of the sur- 

 roundings, they can be made to migrate in an electric field to the 

 anode or to the cathode according to their charge, and it is possible 

 in this way to concentrate them by electrophoresis. 



None of these methods of indirect particle identification, however, 

 is quite as convincing as ultramicroscopj, which makes the particles 

 visible (SiEDENTOPF and Zsigmondy, 1903). Admittedly, the ultra- 

 microscope does not give a true image of the colloid particles, for the 



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