VOYAGE TO ALGERIA 183 



ing the thickness of the stratum we may absorb the whole 

 of the light. The color of a blue liquid is similarly ac- 

 counted for. It first extinguishes the red; then, as the 

 thickness 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 

 complete, 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 ther- 

 mal rays are in part visual, the visual rays in part chem- 

 ical, 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 evap- 

 oration. At the same time the whole spectrum suffers en- 

 feeblement; 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, yellow follows orange, green fol- 

 lows 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 ap- 



