AI'PENDIX 585 



apparent than real and is probably due to tlie limitation of our 

 means of observation. In any system there is a tendency for 

 equilibrium to be established and this is of general importance in 

 science. 



The Tyndall Effect. — Another difference between a homogene- 

 ous system and a heterogeneous one containing very small dis- 

 persed particles can be demonstrated ])y passing a 1)eam of liglit 

 through them and making observations from a position at right 

 angles to the beam of light. A true solution, which is an homoge- 

 neous system, should be clear, wherea.s the colloidal solution, which 

 is an heterogeneous system, would appear turbid. Tlii.s phenom- 

 enon is well illustrated by passing a beam of sunlight through a 

 very small opaning into a darkened room. An observer within the 

 room can see the beam of light extending across the room owing 

 to the presence of dust particles in the air. The light is scattered 

 and part of it is completely polarized. The beam of light from a 

 searchlight is also a good example of the scattering and polarizing 

 of light by fine particles in suspension. This phenomenon, in- 

 volving polarization, was first descrilicd by Faraday (1857) and 

 later by Tyndall in 1869. It is usually called the Tyndall Effect. 



Colloids are usually divided into two classes, suspensoids and 

 emulsoids. 



Suspensoids and Emulsoids. — Suspensoids. — In these the dis- 

 persed phase is merely suspended in the dispersion medium and 

 does not change the viscosity of the latter. 



Emulsoids. — Here the dispersion medium and dispersed phase 

 enter into such intimate relationship with each other that the 

 viscosity is changed. 



Transferability of Emulsoid to Suspensoid. — It is iiossible to 

 convert an emulsoid into a suspensoid, as, e.g., when 50 per cent 

 ammonium chloride is added to blood serum, which is a complex 

 emulsoid, sufficient water is witlidrawn from the .serum globulin, 

 present as a dispersed phase, to change it from an emulsoid to a 

 suspensoid, and hence we say it is salted out. 



Chemical vs. Colloid Reactions. — It is important to note that 

 material in the colloidal state exhibits properties quite different 

 from those when in true solution. When substances in true solu- 

 tions react with each other, they do so according to known chem- 



