NATURE 
THURSDAY, OCTOBER 20, 1870 
ON THE COLOUR OF THE LAKE OF GENEVA 
AND THE MEDITERRANEAN SEA 
HROUGH kindness for which I have reason to feel 
both proud and grateful, I have had placed in my 
hands two bottles of water taken from the Mediterranean 
Sea, off the coast of Nice. To my friend M. Soret I 
am also indebted for two other bottles taken from the 
Lake of Geneva. The friendly object in each case was to 
enable me to examine whether the colour of the water 
could in any way be connected with the scattering of light 
by minute foreign particles, which is found so entirely 
competent to produce and explain the colour of the sky. 
In the open Lake of Geneva, Soret himself had studied 
this question with considerable success,* and my desire 
was to apply to it other methods of examination. 
The bottles, as they reached me, and with their stoppers 
unmoved, were placed in succession in the convergent 
beam of an electric lamp. Water optically homogeneous 
would have transmitted the beam without revealing its 
track. In such water the course of the light would be no 
more seen than in optically pure air. The cone of light, 
however, which traversed the liquid, was in both cases 
distinctly blue, the colour from the Lake of Geneva water 
being especially rich and pure. Something, therefore, 
existed in the liquid which intercepted and scattered, in 
excess, the shorter waves of the beam. The longer waves 
were also scattered, but in proportions too scanty to render 
the track of the beam white. The action, in fact, was 
identical with that of the sky. Viewed through a Nicol’s 
prism the light was found polarised, and the polarisation 
along the perpendicular to the illuminating beam was a 
maximum. In this direction, indeed, the polarisation 
was sensibly perfect. A crystal of tourmaline placed 
with its axis perpendicular to the beam was transpa- 
rent ; with its axis parallel to the beam it was opaque. 
By shaking the liquid larger particles could be caused to 
float and sparkle in the beam. The delicate blue light 
between these particles could be quenched by the Nicol 
while they were left shining in the darkened field. A 
concave plate of selenite, placed between the Nicol and 
the water, showed a system of vividly coloured rings. 
They were most brilliant when the vision was at right 
angles to the beam, just as they are most brilliant when 
the blue sky is regarded at right angles to the rays of 
the sun. Inno respect could I discover that the blue of 
the water was different from that of the firmament. The 
colour presented by the Mediterranean water was a good 
sky-blue, while that presented by the Geneva water 
matched a sky of exceptional purity. 
My interest was long ago excited by the attempts made 
to account for the colour of the Lake of Geneva, and con- 
tinued observation in 1857 impressed me more and more 
with the notion that the blue was mainly that of a turbid 
medium, Soon afterwards I wrote thus regarding this 
colour :— 
“Tsit not probable that this action of finely divided 
matter may have some influence on the colour of some cf 
the Swiss lakes—on that of Geneva for example? This 
* See his letter to me, Philosophical Magazine, May, 1869, 
VOL. Il. 
489 
lake is simply an expansion of the river Rhone, which 
rushes from the end of the Rhone glacier. Numerous 
other streams join the Rhone right and left during its 
downward course, and these feeders being almost wholly 
derived from glaciers, carry with them the fine matter 
ground by the ice from the rocks over which it has 
passed. Particles of all sizes must be thus ground off, 
and I cannot help thinking that the finest of them 
must remain suspended in the lake throughout its entire 
length. Faraday has shown that a precipitate of gold 
may be so fine as to require a month to sink to the bottom 
of a bottle five inches high; and in all probability it 
would require ages of calm subsidence to bring all the 
particles in the Lake of Geneva to its bottom. It seems 
certainly worthy of examination whether such particles, 
suspended in the water, do not contribute to that magnifi- 
cent blue which has excited the admiration of all who 
have seen it under favourable circumstances.” * 
Through the observations of Soret, and through those 
here recorded, the surmise of thirteen years ago has be- 
come the verity of to-day. 
But though in the action of small particles we have a 
cause demonstrably sufficient to produce the blueness 
referred to, it is not the only cause operative. In the 
Lake of Geneva we have not only the blue of scattering 
by small particles, but also the blue arising from true 
molecular absorption. Indeed, were it not for this, the 
light ¢ransmitted by a column of the water would be 
yellow, orange, or red, like the light of sunrise or sunset. 
Not only then is the light mainly blue from the first 
moment of its reflection from the minute particles, but 
the less refrangible elements which always accompany 
the blue are still further abstracted during the transmis- 
sion of the scattered light. Through the action of both 
these causes, scattering and absorption, the intense and 
exceptional blueness both of the Lake of Geneva and 
the Mediterranean Sea I hold completely accounted for. 
During the year 1869, M. Lallemand communicated 
to the Paris Academy of Sciences some interesting 
papers on the optical phenomena exhibited by certain 
liquids and solids when illuminated like the actinic clouds 
in my experiments. I also, in 1868, had examined a great 
number of liquids in the same manner, and a brief refer- 
ence to these experiments will be found towards the end 
of a paper on the blue colour of the sky and the polarisa- 
tion of its light, published in the Proceedings of the Royal 
Society for the 16th of December, 1868. M. Lallemand 
supposed the scattering of the light to be effected not by 
foreign particles but by the molecules of the liquids 
with which he experimented. M. Soret, on the other 
hand, contends against this novel view, maintaining that 
the scattering of the light is an affair of particles and 
not of molecules. While admiring the skill and learn- 
ing displayed by the young French physicist, I am 
forced to take the side of Soret in this discussion 
M. Lallemand assumes a purely hypothetical cause 
while a true cause is at hand. He bases his case 
mainly on clear glass and distilled water. But the 
clearness is that observed in ordinary daylight, which 
is a very deceptive test. Glass exhibits the phenomena 
* Glaciers of the Alps (1860), p. 26r. : 
+ In fact, we hav: a dichroitic action of th's kind exerted by glacier 
water when the subsi lence is lesscomplete than in the Lake of Geneva, 
cc 
