122 PHYSICAL SCIENCE 



molecular weight, or they may be considered to be 

 systems of two phases, composed of suspensions of 

 particles In the liquid, the particles being different 

 in kind from the liquid, and of much greater than 

 molecular dimensions. 



In some colloid solutions the presence of sus- 

 pended particles can be detected readllyby ordinary 

 means. Sometimes they are visible under a good 

 microscope ; In other cases, while too small to be 

 directly visible, they are large enough to scatter 

 and polarise a beam of light. This means that their 

 size must be comparable with the wave-length of 

 light, about 5 x io~^ cm. Such particles would be 

 too few in number to exert a measurable osmotic 

 pressure, and the absence of such pressure does 

 not necessarily mean that solutions of colloids are 

 different in kind from solutions of crystalloids. 



It is worthy of note that turbid suspensions 

 of clay, kaolin, etc.. In water are rapidly cleared 

 by the addition of small quantities of metallic 

 salts. This action, which is almost certainly of 

 the same nature as the coagulation described 

 above, probably helps in the formation of sand- 

 banks at the mouths of rivers ; the salts of the 

 sea-water clear the suspensions of clay brought 

 down with the fresh water, and precipitation is 

 then aided by the diminished velocity. 



The conditions which determine the colloid or 

 crystalloid nature of a substance are still not fully 

 understood. The persistence of colloid properties, 

 when a substance passes from the dissolved to the 

 non-dissolved state, shows that the determining 

 conditions must be of fundamental importance. 

 The molecular forces seem to be much less active 

 in colloids, but the freedom with which some of 



