n8 
On Water and Air. 
[February, 
Bay of Biscay water was almost wholly free from them. In 
the green water the particles were very fine, but so numerous 
as to send an apparently unbroken green light to the eye. 
In deep water the particles rest tranquilly at the bottom, in 
shallow or shoal water they are stirred up by the agitation 
of the sea. Shoal water, therefore, is always green to the 
eye. Thus we connect the colours of the sea with its me- 
chanically suspended matter. 
The following table clearly shows the connection between 
the colour of the sea and its suspended matter : — 
No. 
Locality. 
Colour of Sea. 
Appearance in Luminous Beam. 
1. 
Gibraltar Harbour . . 
Green . . 
Thick, with fine particles. 
2. 
Two miles from Gibraltar 
Clearer green 
Thick, with very fine particles. 
3- 
Off Cabreta Point . . 
Bright green 
Still thick, but less so. 
4. 
Off Cabreta Point . . 
Black-indigo 
Much less thick, very pure. 
5. 
Off Tarifa 
Undecided . . 
Thicker than No. 4. 
6. 
Beyond Tarifa 
Cobalt-blue 
Much purer than No. 5. 
7. 
Twelve miles from Cadiz 
Yellow-green 
Very thick. 
8. 
Cadiz Harbour 
Yellow-green 
Exceedingly thick. 
9. 
Fourteen miles from Cadiz 
Yellow-green 
Thick, but less so. 
10. 
Fourteen miles from Cadiz 
Bright green 
Much less thick. 
11. 
Between Capes St. Mary 
and Vincent 
Deep indigo 
Very little matter, very pure. 
12. 
Off the Burlings . . 
Strong green 
Thick, with fine matter. 
13- 
Beyond the Burlings .. 
Indigo.. 
7 ery little matter, pure. 
14. 
Off Cape Finisterre 
Undecided. . 
Less pure. 
15- 
Bay of Biscay 
Black-indigo 
Very little matter, very pure. 
16. 
Bay of Biscay 
Indigo.. 
Very fine matter, iridescent. 
17- 
Off Ushant 
Dark green . . 
A good deal of matter. 
18. 
Off St. Catherine’s 
Yellow-green 
Exceedingly thick. 
19. 
Spithead 
Green . . 
Exceedingly thick. 
Now I want to show you the effeCt of this suspended 
matter, and I cannot do it better than by showing the effedt 
of our beam upon the dust of this room. The atmosphere 
always contains some suspended matter, and even rain-water 
and the clearest ice are not absolutely pure. 
[In illustration of the purity of water taken from melted 
ice, the apparatus (Fig. 5) was shown and explained. A 
glass funnel, passing air-tight through a glass plate, has 
attached to it a glass bulb B, fitted with a stopcock. A block 
of ice, of great purity, had some days previously been placed 
in the funnel, and covered air-tight by a glass receiver. 
This receiver was connected to an air-pump, and the air 
removed from the interior of the apparatus. Air was then 
allowed to enter the apparatus by passing slowly through a 
tube containing cotton-wool. By this means the entering 
air was completely filtered of its floating matter. The ice 
melted, and the water trickled down the funnel into the 
bulb. When the latter was full the stopcock was opened, 
