136 future's 



ing surfaces that are cast upon the panes of 

 glass. 



If now there were no atmosphere or, to 

 state it differently if there were no floating 

 substances in the atmosphere, the sun would 

 produce an effect upon the earth similar to 

 that of a single electric light. The lights 

 would be extremely high, and the shadows ex- 

 tremely dense. To one looking off into space, 

 the sky, instead of having the blue appearance 

 that we see, would have the effect of looking 

 into a deep, dark abyss without illumination. 



Tyndall has shown us by a beautiful experi- 

 ment that if there be in a glass tube a mixture 

 of gases related to each other in a certain way 

 chemically, they will combine into small 

 globules or particles similar to moisture in the 

 . air. If now a beam of light is thrown upon 

 this tube and a dark screen put behind it, we 

 shall, in the beginning of the experiment, 

 simply see the dark screen. As soon, however, 

 as the molecules of the gases have combined 

 in sufficient numbers to produce particles of 

 sensible size we begin to have a reflection of 

 light from them, the color of which is con- 

 stantly changing as the combining particles 

 grow in size. At a certain stage in its prog- 

 ress the color which the mixture of gases 

 assumes is a beautiful azure blue, rivaling in 

 purity the finest skies of Greece or southern 

 Italy. 



