NIAGARA. 191 



in 1872-73 I had frequent opportunities of testing the 

 explanation there given. Looked properly down upon, 

 there are portions of the ocean to which we should 

 hardly ascribe a trace of blue; at the most, a mere hint 

 of indigo reaches the eye. The water, indeed, is prac- 

 tically black, and this is an indication both of its depth 

 and of its freedom from mechanically suspended mat- 

 ter. In small thicknesses water is sensibly transparent 

 to all kinds of light; but, as the thickness increases, 

 the rays of low refrangibility are first absorbed, and 

 after them the other rays. Where, therefore, the water 

 is very deep and very pure, all the colours are absorbed, 

 and such water ought to appear black, as no light is 

 sent from its interior to the eye. The approximation 

 of the Atlantic Ocean to this condition is an indication 

 of its extreme purity. 



Throw a white pebble into such water; as it sinks 

 it becomes greener and greener, and, before it disap- 

 pears, it reaches a vivid blue-green. Break such a 

 pebble into fragments, each of these will behave like 

 the unbroken mass; grind the pebble to powder, every 

 particle will yield its modicum of green; and if the 

 particles be so fine as to remain suspended in the water, 

 the scattered light will be a uniform green. Hence the 

 greenness of shoal water. You go to bed with the 

 black Atlantic around you. You rise in the morning, 

 find it a vivid green, and correctly infer that you are 

 crossing the bank of Newfoundland. Such water is 

 found charged with fine matter in a state of mechan- 

 ical suspension. The light from the bottom may some- 

 times come into play, but it is not necessary. A storm 

 can render the water muddy, by rendering the particles 

 too numerous and gross. Such a case occurred towards 

 the close of my visit to Niagara. There had been rain 

 and storm in the upper lake-regions, and the quantity 



