] 30 VRA 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 rays, 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 that certain Welsh tarns, which are 

 reputed bottomless, have this inky hue. 



