70 PHYSICAL CHEMISTRY 



Colloids 

 Although Graham defined colloids as a class of non-diffusible, 

 non-crystallizable substances, Von Vietnam claims that any sub- 

 stance may be made colloidal, and hence we should more cor- 

 rectly speak of the colloidal state. Many constituents of cells, 

 however, are practically always in the colloidal state and are 

 conveniently called colloids (emulsoids). Although some pro- 

 teins may be crystallized, the crystals contain water and water 

 soluble substances and are therefore not so very different from 

 gels. The colloids are divided into suspensoids and emulsoids. 



Suspensoids 



The suspensoids, or suspension colloids, are ultramicroscopic 

 suspensions. The particles are said to be liquid when first 

 formed, but some solidify later. With the ultramicroscope the 

 particles may be seen and their size estimated. The ultramicro- 

 scope is merely a microscope with a very powerful dark field 

 illumination. The colloidal particles look like stars in the night 

 except that they may appear colored, depending on their size, 

 the smallest appearing blue. This light dispersed by colloid 

 solutions causes them to opalesce. The light may not only be 

 colored, but polarized as well, a phenomenon named after the 

 discoverer, Tyndall. Even cane sugar and raffinose solutions 

 show the Tyndall effect (De Bruyn, 1900). 



Whereas with the directly illuminated microscope particles 

 .14 n in diameter may be seen, with the ultramicroscope we may 



