On Water and Air . 
1880.] 
117 
tain why one water should be green and another almost 
black, or, at the least, of a shade of indigo. A very good 
fellow, named Thorogood, was lent to me by Capt. Sander- 
son, who was the commander of the vessel in which I sailed. 
The man who was accustomed to heave the lead, used to 
<r e t into a boat near the front of the vessel, and I went 
towards the stern, and this man then used to heave the plate 
out. Being weighted it sank rapidly, and by the time it 
came to the stern, where I was standing, it was at a good 
depth. Immediately after it entered the water it became 
„reen • that green deepened, and approached a black ; but 
I never saw it properly blue. It was a blue-green at the 
mo<=t and that was at the utmost depth. That, then, was 
the colour of the water at the particular depth to which the 
^ Now I think that you will be able to follow me. The 
water at a certain depth coloured the plate of a vivid green. 
Supposing that that plate had been broken into fragments, 
and the fragments thrown into the water, each fragment 
would have sent a modicum of green light to the eye ; there 
would be seen a number of green spots in the water. Sup- 
pose that, instead of being broken into small fragments, the 
plate had been ground into a powder so fine that it would 
remain suspended in the water; every particle of that pow- 
der would send its modicum of light to the eye, and the 
consequence would be that the water with such particles 
diffused through it would appear to be green water You 
see bv the experiment which I made in the blue-black water 
in the Bay of Biscay, that I produced a green colour by 
means of the white plate. I am now going on, by the pro- 
cess of reasoning, to deduce the fadt that if I ground the 
plate into powder, and diffused that powder through the 
water, the water would appear green by reason of the light 
reflected by the little particles of powder to the eye. 
Now this completely accounts for the colours ot the ocean 
observed between Gibraltar and Spithead. It was some- 
times a dull yellow-green, sometimes a bright emerald-green, 
sometimes a cobalt-blue, sometimes an indigo colour, and 
sometimes almost as black as ink. I took samples of all 
these waters in clean stoppered bottles, and by sending a 
powerfully concentrated beam through them, here, in Lon- 
don examined their condition. The blackest water, which 
was taken from the deepest portions of the Bay of Biscay, 
was the purest. After it, in order of purity, came the indigo, 
cobalt, bright green, and yellow-green water. The water 
last named was thick with suspended particles, while the 
