CHAP. ix. THE IMPORTANCE OF DUST. 71 



generally known that if there was absolutely no dust in 

 the air the path of the sunbeam would be totally black 

 and invisible, while if only very little dust w T as present 

 in very minute particles the air would be as blue as a 

 summer sky. 



This was proved by passing a ray of electric light 

 lengthways through a long glass cylinder filled with air 

 of varying degrees of purity as regards dust. In the 

 air of an ordinary room, however clean and well venti- 

 lated, the interior of the cylinder appears brilliantly 

 illuminated. But if the cylinder is exhausted and then 

 filled with air which has passed slowly through a fine 

 gauze of intensely heated platinum wire, so as to burn 

 up all the floating dust particles, which are mainly 

 organic, the light will pass through the cylinder without 

 illuminating the interior, which, viewed laterally, will 

 appear as if filled with a dense black cloud. If, now, 

 more air is passed into the cylinder through the heated 

 gauze, but so rapidly that the dust particles are not 

 wholly consumed, a slight blue haze will begin to appear, 

 which will gradually become a pure blue, equal to that 

 of a summer sky. If more and more dust particles are 

 allowed to enter, the blue becomes paler, and gradually 

 changes to the colorless illumination of the ordinary air. 



The explanation of these phenomena is that the num- 

 ber of dust particles in ordinary air is so great that they 

 reflect abundance of light of all wave-lengths, and thus 

 cause the interior of the vessel containing them to ap- 

 pear illuminated with white light. The air which has 

 passed slowly over white-hot platinum has had the dust 

 particles destroyed, thus showing that they were almost 

 wholly of organic origin, which is also indicated by their 



