286 THE COLLOIDAL STATE 



which may be regarded as a state midway between a true 

 solution and a suspension. The evidence of the ultramicro- 

 scope* goes to support this view. 



In a true suspension the particles are of varying size, but 

 even the smallest are visible under the magnification of a high- 

 power microscope, the limits of visibility of which are some- 

 where of the order of cri //,, in which //, = 'OOI mm. or I 

 millionth part of a meter. The particles of a colloidal solution, 

 on the other hand, may vary between the limits cri //, and 

 )&//,; f such particles, although beyond the limits of direct 

 visibility by the microscope, can nevertheless be revealed in- 

 directly by means of the ultramicroscope, the principle of 

 which is to detect the presence of particles by the light re- 

 flected from them in a dark field in much the same way as a 

 J}eam of sunlight entering a dark room reveals the presence of 

 dust particles by reflected light. 



When the particles in a solution are of a smaller diameter 

 than ftp the}/ are no longer detectable by the ultramicroscope, 

 and in a true solution they are assumed to have diameters of 

 the order of cri p/i the molecule of hydrogen being calculated 

 as having a diameter of O'i6 pp. 



It may be assumed then that in colloidal solutions we are 

 dealing with non-homogenous mixtures or two phase systems, 

 and that the characteristic properties of such colloidal solutions 

 are attributable to this peculiar state of aggregation. 



This explains at once why the rate of diffusion of sub- 

 stances in colloidal solution should be slower than those in 

 true solution, since the larger particles would naturally be 

 expected to move more slowly than the particles of molecular 

 , dimensions found in true solution. Moreover, it accounts for 

 the low values obtained in the measurement of the osmotic 

 pressure of colloids by the freezing-point method. 



If osmotic pressure is ultimately caused by the impact of 

 particles upon the walls of the containing vessel, then the 

 more sluggish larger particles would produce fewer impacts 

 and therefore a lower osmotic, pressure than the more rapidly 

 moving particles of molecular dimensions. Direct determina- 



* For a description of this apparatus and its use see Zsigmondy : " Colloids 

 and the Ultramicroscope ". Trans, by Alexander. New York, 1909. 

 t /i/x = 'ooi /j. = i millionth part of a millimeter. 



