258 THE POPULAR SCIENCE MONTHLY. 



mountain than from a valley. To this matter Mr. Aitken has 

 been giving some attention, and his observations point the very 

 opposite way, corroborative of his dust-theory. From the summit 

 of the Rigi Kulm in Switzerland he saw several sunsets, but was 

 disappointed with the flatness and weakness of the coloring ; 

 whereas in the valley, on the same evenings, careful observers 

 were enchanted with the gorgeous display. The lower dusty 

 humid air was the chief source of the color in the sunset effects. 

 His opinon is strengthened by the fact that when from the summit 

 he saw large cumulous clouds, the near ones were always snowy 

 white, while it was only the distant ones that were tarnished 

 yellow, showing that the light came to these clouds unchanged, 

 .and it was only the air between the far-distant clouds and his eye 

 that tarnished them yellow. On the mountain-top it required a 

 great distance to give even a slight coloring. The larger and 

 more numerous dust-particles in the air of the valley are, therefore, 

 productive of more brilliant coloring in sunrise or sunset than 

 the smaller and fewer particles on the mountain-top. 



It is now admitted that the inherent hue of water is blueness. 

 Even distilled water has been proved to be almost exactly of the 

 same tint as a solution of Prussian blue. This is corroborated by 

 the fact that the purer the water is in nature, the bluer is the hue. 

 But though the selective absorption of the water determines its 

 blueness, it is the dust-particles suspended in it which determine 

 its brilliancy. If the water of the Mediterranean be taken from 

 different places and examined by means of a concentrated beam 

 of light, it is seen to hold in suspension millions of dust-particles 

 of different kinds. To this fine dust it owes its beautiful, brilliant, 

 and varied coloring. Where there are few particles there is little 

 light reflected, and the color of the water is deep blue ; but where 

 there are many particles more light is reflected, and the color is 

 chalky blue-green. Along its shores the Mediterranean washes 

 the rocks and rubs off the minute solid particles, which make the 

 water beautifully brilliant. 



That this is the case can be illustrated. If a dark metal vessel 

 be filled with a weak solution of Prussian blue, the water will 

 appear quite dark and void of color. But if some fine white 

 powder be thrown into the vessel, the water at once becomes of a 

 brilliant blue color ; if more powder be added, the brilliancy in- 

 creases. This accounts for the changes of depth and brilliancy of 

 color in the several shores of the Mediterranean. In Lake Como, 

 where there is an entire absence of white dust-particles, the water 

 is of a deep blue color, but void of brilliancy ; but, where the 

 lake enters the river Adda, the increase of the current rubs down 

 fine reflecting particles from the rocks ; in consequence, there the 

 water is of a finer blue. When the dust-particles carried down 



