NIAGARA. 243 



planation 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 practically black, and this 

 is an indication both of its depth and of its freedom from 

 mechanically suspended matter. 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 mechanical 

 suspension. The light from the bottom may sometimes 

 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 of 

 suspended matter brought down quite extinguished the 

 fascinating green of the Horseshoe. 



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