Stranica 123 [123]
OCR
COLOURS IN STILL WATER 6 water. If the water is deep enough, the stone disappears, for the blue rays, though not absorbed by the water so quickly as the others, cannot pierce the great distance from the surface to the bottom and back again. So that if the water is clear, that is to say, if it carries no particles to reflect light to the eye, and is sufficiently deep, it will appear black. But in nature water is seldom quite clear, and light is usually reflected upwards from suspended particles, and if shallow, from stones at the bottom also. Thus it is that we see the colour of water. Light from above enters the water and is reflected to the eye from the minute particles floating in it ; in passing through the water, certain of its constituent rays are absorbed in greater proportion than others, so that it has on exit a predominance of blue or green rays. The resultant colour depends of course to some extent on the colour of the particles themselves. White particles floating in pure water? make it appear blue, whilst yellow particles give it a greenish tint. In lakes the blackness is probably, in most cases, due to yellow stain, which stops all the light escaping the absorption of pure water. 2 Without going here into the question of the inherent colour of water, it may be stated generally that pure water is of a beautiful blue colour, very closely resembling that of Prussian blue. When stained with peat, or other vegetable matter, it becomes first green, then yellowish-brown. ‘Thus the difference in colour, often so marked, of the Swiss lakes is accounted for, the bluest lakes being the purest, whilst the greener lakes contain a small quantity of vegetable matter in solution. Again, with regard to the seas surrounding our islands, though their greenness may be in part due to the yellow colour of the minute particles of floating sand, it 15 probably also largely caused by the influx of yellowish-brown river
