VOYAGE TO ALGERIA. 171 



nary remark or two will clear our way towards an ex- 

 planation. Colour resides in white light, appearing 

 when any constituent of the white light is withdrawn. 

 The hue of a purple liquid, for example, is immedi- 

 ately accounted for by its action on a spectrum. It 

 cuts out the yellow and green, and allows the red and 

 blue to pass through. The blending of these two 

 colours produces the purple. But while such a liquid 

 attacks with special energy the yellow and green, it 

 enfeebles the whole spectrum. By increasing the thick- 

 ness of the stratum we may absorb the whole of the 

 light. The colour 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 vis- 

 ual, 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 at- 

 tacks all its rays, but with different degrees of energy. 



never seen the Lake of Geneva, but I thought of the brilliant 

 dazzling dark blue of the mid-Atlantic under the sunlight, and 

 its black-blue under cloud, both so solid that one might leap off 

 the sponson on to it without fear ; this was to me the most won- 

 derful thing which I saw on my voyages to and from the West 

 Indies.' 



