THE COLLOIDAL STATE 105 



Rayleigh believed the colors due to multiple thin films 

 separated by air. 



The blue color of the sky and the deep sea has been the subject 

 of much discussion. Rayleigh suggested, and the theory is 

 now generally accepted, that the blue of the sky is due to the 

 scattering of the short blue rays of light by the gas molecules 

 of the atmosphere. In other words, there is a molecular scatter- 

 ing of light comparable to that produced by colloidal particles. 

 Raman discovered that molecular diffusion of light occurs 

 also in water, and therefore the blue of the sea can be explained 

 in this way. There is, however, evidence enough to support 

 the contention that suspended, colloidal matter is largely 

 responsible for the color of the sea, as it is for the blue of smoke, 

 fog, snow, and many colloidal solutions. 



Brownian Movement. — In the September, 1828, issue of the 

 Philosophical Magazine, Robert Brown published the fact that 

 he had observed in germinating pollen grains the tremblmg 

 motion of particles measuring about 0.004 to 0.005 in. in size. 

 He remarked that their motion resembled *4n a remarkable 

 degree the less rapid motion of some of the simplest animalcules 

 of infusion." Thus did there arise in his mind the question 

 whether or not this extraordinary activity could be a vital one. 

 Since he had found it in living plants, he investigated twenty- 

 year-old herbarium specimens, which could not possibly have 

 any life in them, and found there the same motion of minute 

 granules. He thought that these particles were possibly the 

 "elementary molecules" of organic substances. But he dis- 

 proved this speculation himself when he later found that 

 "rocks of all ages including those in which organic remains 

 had never been found yielded the molecules in abundance." 

 We now know that what Brown saw was the active motion 

 that bears his name, the Brownian movement of colloidal 

 particles. 



Molecules, because of their innate kinetic energy, are in an 

 active state of movement. The movement is not a vibration 

 but a hither-and-thither or zigzag one in which the molecules 

 go from place to place within a relatively small region, though 

 they may wander quite far, without ever returning to the original 

 position. The motion is not to be confused with the diffusion 

 of molecules in solution, though the one (Brownian movement) 



