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 increases. 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
