122 SCIENCE PROGRESS 



although of much interest, surface phenomena are not strikingly 

 noticeable, except in the case of colloids. Here the specific 

 surface is so enormous that surface effects transcend in import- 

 ance the intrinsic properties of the substances concerned. Thus 

 in the study of colloids we come into contact with the tremendous 

 forces experienced by the molecules of matter. Indeed it may 

 be said that colloids are matter in the nascent state. No 

 wonder, therefore, that their properties should be astonishing. 



So far, two types of colloid condition have been studied in 

 detail : sols, or colloid solutions, and gels, or jellies. Other 

 types, as glasses and resins, appear to resemble gels, but differ 

 in being formed from a cooling melt and having no liquid phase. 

 Others, again, as starch, charcoal, and cellulose, have their 

 interstices filled with air instead of liquid. 



Colloid solutions, or sols, are distinguished by the presence 

 of particles of a size intermediate between those of ordinary 

 solutions and those of suspensions of solid matter in liquids. 

 In so-called true solutions the particles of dissolved substance 

 are comparable in magnitude with those of the solvent. 

 Doubtless there is a field of force surrounding each dissolved 

 particle, but these are so small that it seems impossible to 

 detect their individual existence, and the solution appears to 

 be practically homogeneous. On the other hand, although 

 the dispersed particles in sols are still too small to be seen 

 directly with the most powerful microscope, yet their size is 

 sufficient to bring them within the range of the more delicate 

 apparatus of modern research. 



When an extremely powerful beam of light is transmitted 

 through a colloid solution at right angles to the axis of the 

 microscope, the light is scattered by the dispersed particles, 

 some of it enters the microscope tube, and the particles are 

 seen as points of light against a dark background. Such an 

 arrangement is called an ultramicroscope. The appearance 

 presented is indeed amazing. Thousands of tiny particles, 

 bright as sparks of emerald or ruby, or shining like gold, are 

 seen to be dancing like gnats in a sunbeam. They hop, jump, 

 dash together and fly away again, and the motion continues 

 for ever. It has been demonstrated that the effect is not due 

 to vibration or to any external cause. Here for the first time 

 we have visible evidence of the endless dance of the atoms 

 that will cease only when the heat energy of the universe is 

 completely dissipated and all matter is cooled to the absolute 

 zero of temperature. The colloidal particles are small enough 

 to exhibit, in less degree, the attributes of molecules themselves. 

 And this is the great fascination of colloid chemistry, that in 

 the conduct of colloid particles we can trace the behaviour of the 

 invisible molecules. Thus, the energy of motion of the particles 



