Stranica 123 [123]
COLOURS IN STILL WATER 6
water. If the water is deep enough, the stone dis¬
appears, 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 sur¬
rounding 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