SPONTANEOUS GENERATION 101 



which light is affected by minute particles in the 

 air and other media which it traverses. The blue- 

 ness of the sky and the blue color of the sea and 

 the water of deep lakes he explained as due to 

 minute particles held in suspension which, on ac- 

 count of their very small size, reflect chiefly only 

 the shortest light waves of the visible spectrum, 

 or those near the violet end. It was necessary for 

 him in the course of his experiments to obtain air 

 free from floating matter. Light passing through 

 ordinary air reveals its course by being reflected 

 from a multitude of minute particles. Tyndall 

 showed that a beam of light sent through air free 

 from floating particles is absolutely invisible. I 

 have often noticed in the shade of the dense red- 

 wood forests of northern California how beams of 

 light reveal their path, high up among the trees, 

 by delicate hazy streaks in the exceptionally pure 

 air of that region. Even there the air is charged 

 with floating matter. Of what does it consist? 



Tyndall studied the question in samples of the 

 air of London by an ingenious method. A coil of 

 platinum wire which could be heated to redness by 

 an electric current was enclosed in a glass vessel 

 into which air could pass only by going through a 

 dense plug of cotton. A beam of light passing 



