of Edinburgh, Session 1881 - 82 . 
475 
all colours, and the water, by its selective absorption, strikes down 
the red rays, and only the blue rays are reflected to the surface and 
to the eye. These solid particles determine the brilliancy, and the 
selective absorption of the water determines its colour. 
The colour and the amount of the suspended particles is then 
considered. It is shown that the colour of the particles will have 
a marked influence in the appearance of the water. If the particles 
are yellow — sand particles for instance — then a blue coloured water 
will appear to be green, as the light reflected by the yellow particles 
is deficient in the rays of the violet end of the spectrum. 
In the Mediterranean the solid particles are whitish, and all the 
different colour phenomena are easily explained by the different 
amounts of the reflecting particles at the different places. Where the 
colour is deep blue there are few particles in the water, and but 
little light reflected ; and, further, the light passes through a great 
amount of water, and undergoes a great amount of selective absorp- 
tion before it is reflected to the surface and to the eye. But if 
there are many particles in the water much light is reflected, and 
the colour is chalky blue-green, as the light does not pass through so 
great a depth of water, and is therefore not so deeply coloured, nor 
has it so many of the green rays cut out as in the water where the 
particles are few and far separated. 
Colour experiments on a small scale with a solution of Prussian 
blue and a fine white powder are described. If the solution of 
Prussian blue is placed in a vessel, the bottom and sides of which 
are dark and reflect no light, then the coloured solution appears 
dark and colourless ; but if a little of the white powder is added 
then the solution at once becomes brilliantly coloured. By varying 
the amount of the powder in the water all the varied colour effects 
of the Mediterranean can be reproduced, a little powder causing 
the solution to appear deep blue, and as more powder is added the 
brilliancy of the water increases, and its colour changes from blue 
to chalky blue-green. 
The presence and the abundance of white reflecting particles is 
shown to be a characteristic of all finely coloured waters, and the 
wave-washed shores of the Mediterranean are shown to be the 
factories in which are prepared its reflecting particles. The waves, 
as they beat on the shore, grind up the stones and rocks, and stir 
VOL. xi. 3 p 
