xi6 On Water and Air. [February, 
water is perfectly clear. I mean that it is water without 
any mud or suspended matter in it, but still capable of 
colouring light, as was the water which was shown here in 
our last ledture. Well, I say take water pure enough and 
deep enough to allow the whole of the light to plunge 
into it until the whole spedtrum is extinguished. You 
see plainly that that water would not differ from our 
ink. You would have surface refledtion, but you would have 
no light from the interior of the water. The colours of the 
spedtrum would be quenched, and the consequence is that 
you would have pradtically a black liquid, no matter how 
transparent the water might be. Now, if any of you ever 
cross the Atlantic, or even cross the Bay of Biscay, you will 
find that in certain deep portions of the sea the water ap- 
proaches a fine shade of indigo, and it is sometimes almost 
as black as ink. I remember the late most excellent Mr. 
Charles Kingsley writing to me about this. He said that 
nothing impressed him more, in crossing the Atlantic, than 
the wonderful blackness of the sea. But you see that the 
cause of this is that the water is exceedingly pure and ex- 
ceedingly deep, — so deep that the sunlight, when plunged 
into it, is adtually quenched within the body of the water, 
and none is sent back to the eye. The same thing occurs 
pradtically in some of the moraines in the Swiss glaciers, 
where the ice is exceedingly pure and exceedingly deep. 
Looked at superficially that ice is almost as black as pitch. 
Break a piece off it, and look through it, and it is almost as 
transparent as crystal. And so with regard to the Atlantic 
water, which looks so inky black. If you take it in a glass 
you will find that it is as transparent as possible. 
Now, whence come the varying colours of the ocean 
water? You sometimes see it a vivid green, and sometimes 
a yellow-green. In 1870, when I went to Algiers to try to 
observe a total eclipse of the sun, I tried to solve this 
question, and in order to do so I took this white plate that 
you see here before you, which you see is weighted with a 
leaden weight ; and here is a portion of a long rope which 
was used in the experiment. My objedt was to account for 
the extraordinary changes that I observed in the colour of 
the sea. I have observed them since in crossing the Atlantic 
from the banks of Newfoundland. For instance, you pass 
from water which is of this deep indigo, or almost as black 
as ink, into water of a vivid green ; and in going from 
Gibraltar to Spithead the variations in the colour of the 
water were extraordinary. I operated in this way in order 
to instrudt myself upon the subjedt, and in order to ascer- 
