] 30 FRA GMENTS OF SCIENCE. 
ness augments, it attacks the orange, yellow, and green 
in succession; the blue alone finally remaining. But 
even it might be extinguished by a sufficient depth of the 
liquid. 
And now we are prepared for a brief, but tolerably com- 
plete, statement of that action of sea-water upon light, to 
which it owes its darkness. The spectrum embraces three 
classes of rays the thermal, the visual, and the chemical. 
These divisions overlap each other; the thermal rays are in 
part visual, the visual rays in part chemical, and vice versa. 
The vast body of thermal rays lie beyond the red, being 
invisible. These rays are attacked with exceeding- energy 
by water. They are absorbed close to the surface of the 
sea, and are the great agents in evaporation. At the same 
time the whole spectrum suffers enfeeblement; water 
attacks all its ravs, but with different degrees of energy. 
Of the visual rays, the red are first extinguished. As the 
solar beam plunges deeper into the sea, orange follows red, 
yellows follows orange, green follows yellow, and the 
various shades of blue, where the water is deep enough, 
follow green. Absolute extinction of the solar beam 
would be the consequence if the water were deep and 
uniform. If it contained no suspended matter, such 
water would be as black as ink. A reflected glimmer of 
ordinary light would reach us from its surface, as it would 
from the surface of actual ink; but no light, hence no 
color, would reach us from the body of the water. 
In very clear and deep sea-water this condition is 
approximately fulfilled, and hence the extraordinary dark- 
ness of such water. The indigo, already referred to, is, I 
believe, to be ascribed in part to the suspended matter, 
which is never absent, even in the purest natural water; 
and in part to the slight reflection of the light from the 
limiting surfaces of strata of different densities. A modi- 
cum of light is thus thrown back to the eye, before the 
depth necessary to absolute extinction has been attained. 
An effect precisely similar occurs under the moraines of 
glaciers. The ice here is exceptionally compact, and, 
owing to the absence of the internal scattering common in 
bubbled ice, the light plunges into the mass, where it is 
extinguished, the perfectly clear ice presenting an appear- 
ance of pitchy blackness.* 
* I learn from a correspondent tbat certain Welsh tarns, which are 
reputed bottomless, have this inky hue. 
