VOYAGE TO ALGERIA. 129 



the other at Spithead. The sea at both these places was 

 green, and both specimens, as might be expected, were 

 pronounced by the home examination to be thick with 

 suspended matter. 



Two distinct series of observations are here referred to 

 the one consisting of direct observations of the color of 

 the sea, conducted during the voyage from Gibraltar to 

 Portsmouth: the other carried out in the laboratory of the 

 Royal Institution. And here it is to be noted that in the 

 home examination I never knew what water was placed in 

 my hands. The labels, with the names of the localities 

 written upon them, had been tied up, all information 

 regarding the source of the water being thus held back. 

 The bottles were simply numbered, and not till all of them 

 had been examined, and described, were the labels opened, 

 and the locality and sea-color corresponding to the various 

 specimens ascertained. The home observations, there- 

 fore, must have been perfectly unbiased, and they clearly 

 establish the association of the green color with fine sus- 

 pended matter, and of the ultramarine color, and more 

 especially of the black-indigo hue of the Atlantic, with the 

 comparative absence of such matter. 



So much for mere observation; but what is the cause of 

 the dark hue of the deep ocean?* A preliminary remark 

 or two will clear our way toward an explanation. Color 

 resides in white light, appearing when any constituent of 

 the white light is withdrawn. The hue of a purple liquid, 

 for example, is immediately 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 colors produces the purple. But while such 

 a liquid attacks with special energy the yellow and green, 

 it enfeebles the whole spectrum. By increasing the 

 thickness of the stratum we may absorb the whole of the 

 light. The color of a blue liquid is similarly accounted 

 for. It first extinguished the red; then, as the thick- 



* A note, written to me on October 22, by my friend Canon Kings- 

 ley, contains the following reference to this point: " 1 have 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 with- 

 out fear; this was to me the most wonderful thing which I saw on 

 my voyages to and from the West Indies." 



