THE COLLOIDAL STATE 101 



as they, while in suspension, show colloidal properties. Atmos- 

 pheric dust usually settles rather quickly, but some remains 

 long suspended in the air. When the volcanic island Krakatoa 

 blew up, the heavier pieces soon fell, but the finest of the dust 

 particles traveled around the world. The particles of dispersed 

 oil in the emulsions of our common experience are well above 

 the lower limit of microscopic visibility, yet these emulsions 

 are typically colloidal in that they show colloidal properties. 

 So also is it with foams {e.g., soapsuds and sea foam) the dis- 

 persed air bubbles of which are often far above ultramicroscopic 

 (colloidal) dimensions, frequently even of macroscopic size, 

 visible to the unaided eye. Can such systems, therefore, be 

 colloidal? According to a definition based on size of particle, 

 no, but emulsions and foams present the problems of colloidal 

 chemistry. Consequently, a definition based on particle size, 

 while useful, must eventually give way to one based on behavior, 

 for of colloidal systems we ask not what they are but what they 

 do. A substance which exhibits colloidal properties must be 

 regarded as within the field of colloidal chemistry, even if certain 

 other conditions, such as particle size, somewhat arbitrarily 

 laid down, are not met. 



Another problem dependent upon particle size is that of the 

 scattering, or reflection, of light by colloidal particles. The 

 lateral illumination of dust particles in a darkened room differs 

 in one fundamental respect from truly colloidal examples of 

 the Tyndall cone. In the latter case, the particles are not 

 visible as such; they merely appear to be so, while the illumi- 

 nated dust particles in the room are actually visible. This is 

 true because the dust particles reflect the fight, while the colloidal 

 particles scatter it. In order for a particle to act as a mirror, 

 it must be at least as broad as the wave it reflects; in other 

 words, if a surface is to turn back a wave, it must be as large 

 as the wave. The cliffs of Dover will turn back any wave, 

 but a small stick placed at the ocean shore cannot reflect the 

 waves. It breaks them up, or scatters them. This is what 

 happens to light and sound waves when they strike a particle 

 smaller than themselves. The scattering of light is a form of 

 diffraction. Diffraction, or scattering, takes place whenever 

 fight impinges upon an edge or point. Diffraction gratings 

 (parallel lines ruled on glass, as many as 1,000 to a centimeter) 



