102 PROTOPLASM 



cause a beam of light to be diffracted, or broken up, and multiplied 

 many times over (Fig. 168). That colloidal particles do not give 

 rise to reflected wave fronts but act as centers from which 

 light disturbances spread out in all directions is easily proved 

 by a comparison of dimensions. If a mirror must have a diameter 

 at least equal to the size of the wave it is to reflect, then colloidal 

 particles must have a diameter of about 0.5 ji (5,000 A. U.) in order 

 to reflect white light. The visible spectrum lies between 0.39 

 and 75 ^ (3,900 to 7,500 A. U.). The average wave length 

 of white light is, therefore, about 0.5 ^x, which is five times the 

 upper limit of size (0.1 jx) of colloidal particles; consequently, 

 the particles are too small to function as mirrors. 



Now that we are concerned with colloidal particles, let us 

 consider for a moment if the particle need always be an aggregate 

 of molecules, that is to say, if a true particle need always be 

 present. We shall later see that while one group of colloids, 

 the suspensions {e.g., colloidal gold), are characterized by 

 having particles that are aggregates of molecules, the other 

 group {e.g., gelatin) need not necessarily be built up of aggregates; 

 the molecules themselves, because they are very large, satisfy 

 the conditions of particle size. Furthermore, in other systems, 

 particles as such need not even be present for the system to be 

 colloidal. Charcoal has colloidal properties, but neither it 

 nor the substance that it has absorbed (with which its pores are 

 filled) is dispersed, in the true sense, for both phases are con- 

 tinuous. We must, therefore, define colloids as systems in 

 which one substance is distributed discontinuously or con- 

 tinuously in another continuous substance. The definition 

 previously given is, however, the generally accepted one and 

 characterizes nearly all colloidal systems. Furthermore, colloid 

 chemistry has grown up on the basis of the particle concept, 

 and, while there are exceptions, the particle is still the distinguish- 

 ing feature in most cases. It is, however, important to remember 

 that in colloidal systems, exposed surface is the significant thing, 

 whether on a large molecule, a particle, or a continuous surface. 



Color. — The Tyndall cone is often of a bluish tone. Colloidal 

 smoke is blue. Light rain clouds are blue. Snow and ice have 

 a distinct bluish tinge. A trace of soap (which is colloidally 

 dispersed) gives a tint of blue to water. Blue eyes are due to a 

 turbid medium; i.e., the color is colloidal. This predominance 



