70 THE WONDERFUL CENTURY. 



soon as we consider pure air, sunlight, and natural 

 beauty to be of more importance to the population as a 

 whole than are the prejudices or the vested interests of 

 those who produce the smoke. 



But though we can thus minimize the dangers and the 

 inconveniences arising from the grosser forms of dust, 

 we cannot wholly abolish it; and it is, indeed, fortunate 

 we cannot do so, since it has now been discovered that it 

 is to the presence of dust we owe much of the beauty, 

 and perhaps even the very habitability, of the earth we 

 live upon. Few of the fairy tales of science are more 

 marvellous than these recent discoveries as to the varied 

 effects and important uses of dust in the economy of 

 nature. 



The question why the sky and the deep ocean are both 

 blue did not much concern the earlier physicists. It 

 was thought to be the natural color of pure air and water, 

 so pale as not to be visible when small quantities were 

 seen, and only exhibiting its true tint Avhen we looked 

 through great depths of atmosphere or of organic water. 

 But this theory did not explain the familiar facts of the 

 gorgeous tints seen at sunset and sunrise, not only in the 

 atmosphere and on the clouds near the horizon, but also 

 in equally resplendent hues when the invisible sun shines 

 upon Alpine peaks and snowiields. A true theory 

 should explain all these colors, which comprise almost 

 every tint of the rainbow. 



The explanation was found through experiments on 

 the visibility or non-visibility of air, which were made 

 by the late Professor Tyndall about the year 1868. 

 Everyone has seen the floating dust in a sunbeam when 

 sunshine enters a partially darkened room; but it is not 



