REACTIONS IN HETEROGENEOUS SYSTEMS 51 



colloidal solution may be regarded as a solution containing a 

 solute of high molecular weight, or an aggregate of molecules, 

 so that each particle represents a large total molecular weight 

 or, on the other hand, a colloidal solution may be regarded as 

 a suspension of one phase in another so that there are surfaces 

 of separation and the phenomena of surface tension hold at 

 these interfaces. 

 The ultra-microscope shows that colloidal solutions consist 



FIG. 9. Diagram of the course of the rays of light in the ultramicro- 



scope. 

 From "Principles of General Physiology." W. M. BAYLISS (Longmans). 



of particulate matter. This instrument makes use of the 

 Tyndall phenomenon that a bright beam of light causes 

 diffraction haloes around small particles so that viewed at right 

 angles to the beam of light objects too small to be seen by the 

 eye can be detected. By using a microscope at right angles 

 to the beam of light smaller diffraction haloes can be detected, 

 the particles that cause them not being visible under a 

 microscope.* 



The suspension of one phase in another is spoken of as a 



* H. Siedentopf and R. Zsigmondy, Ann. d. Physik., 1903, vol. 10, 

 p. I, 



