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 arid 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the white light is withdrawn.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 vellow 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: " I 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." 
