184 FRAGMENTS OF SCIENCE 



proximately 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 

 appearance of pitchy blackness. 1 



The green color of the sea has now to be accounted 

 for; and here, again, let us fall back upon the sure basis 

 of experiment. A strong white dinner-plate had a lead 

 weight securely fastened to it. Fifty or sixty yards of 

 strong hempen line were attached to the plate. My assist- 

 ant, Thorogood, occupied a boat, fastened as usual to the 

 davits of the "Urgent," while I occupied a second boat 

 nearer the stern of the ship. He cast the plate as a mari- 

 ner heaves the lead, and by the time it reached me it had 

 sunk a considerable depth in the water. In all cases the 

 hue of this plate was green. Even when the sea was of 

 the darkest indigo, the green was vivid and pronounced. 

 I could notice the gradual deepening of the color as the 

 plate sank, but at its greatest depth, even in indigo water, 

 the color was still a blue green.* 



1 I learn from a correspondent that certain "Welsh tarns, which are reputed 

 bottomless, have this inky hue. 



* Iii no case, of course, is the green pure, but a mixture of green and blut. 



