388 THE COLLOIDAL STATE 



It will be seen from the foregoing that whereas in a true 

 solution the dissolved substance is in a state of molecular dis- 

 persion this is not so in what is known as a colloidal solution, 

 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 o-i fi, in which /x = -ooi mm. or i 

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

 on the other hand, may vary between the limits o-i ju, and 

 fxfji 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 

 beam 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 /X./X they are no longer detectable by the ultramicroscope, 

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

 the order of o-i /x/x. — the molecule of hydrogen being calculated 

 as having a diameter of 0-i6 [Mfx. 



It may be assumed then that in colloidal solutions we are 

 dealing with non-homogeneous 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. 



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

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

 1909. 



I /x/x = -001 [1=1 millionth part of a millimeter. 



