THE COLLOIDAL STATE 103 



of blue color appears to be the result of selective scattering. 

 When a beam of white light enters a colloidal dispersion, waves 

 of all sizes impinge upon the particles, but they are not scattered 

 in like proportion. If we return to our cliffs and stick at the 

 seashore, it will be evident that the smallest waves will be 

 scattered (broken up) best by the smaller obstacle. The same 

 is true of light waves. The color of colloidal systems is a phe- 

 nomenon involving a number of factors, and it is difficult to 

 be certain which of these is primarily responsible in a specific 

 case, but the rule that the smaller the particle the greater the 

 scattering of short waves appears to be of general application. 

 To this can be added the fact that the shorter the wave of 

 light the greater the amount scattered. (Stated more specifi- 

 cally, the intensity of the scattered light is directly proportional 

 to the square of the size of the particle and inversely propor- 

 tional to the fourth power of the wave length.) In fine colloidal 

 dispersions, blue, therefore, predominates, in both color and 

 intensity. 



There are a number of color phenomena in nature which 

 may be wholly or partly due to colloidal properties, though 

 other factors usually enter in. What these other factors are 

 and to what extent they play a part in natural colors is not 

 always known. The blue of the deep ocean and of the sky 

 appears to be due primarily to factors other than colloidal 

 ones. Deep water means clear water where only the finest 

 particles are in suspension. If the color of the sea is in part 

 colloidal, then blue would be the expected color. The color 

 of the sky by day is ascribed to the scattering of light by molecular 

 and not colloidal particles, while the color of the sky at twilight 

 is very probably a colloidal phenomenon. 



We view the setting sun along a path close to the surface 

 of the earth where dust is thickest and particles largest. 

 The rays scattered by these particles will be long ones ; red and 

 orange should, therefore, predominate, and blue be absent in 

 sunsets. When the volcanic dust of the exploded island of 

 Krakatoa reached the West Indies, halfway around the world, the 

 whole sky there was an unusually brilliant red throughout the day, 

 as in a sunset, owing to colloidal particles of relatively large size. 



The interpretation of the color of metallic colloidal suspensions 

 is difficult. As gold particles increase in size, the color of the 



