BLUE WATER
Most people know and admire the splendid expanse of blue colour offered
by the clear sea water on many parts of our coasts, and by that of
lakes at home and abroad. I find that there is still a sort of a fixed
determination not to believe that this colour is due (as it is) to the
actual blue colour of pure water. Pure, transparent water is blue.
Those who think they know better will point to a glass of pure water,
hold it up to the light, and affirm that it is colourless. But this
apparent colourlessness is due to the small breadth of water in the
glass through which the light passes. It is definitely ascertained that
if water as pure and as free from either dissolved or suspended matter
as it is possible to make it (by distillation and the use of vessels
not acted upon by water) be made to fill an opaque tube 15 ft. long,
closed at each end by a transparent plate, and then a beam of light be
made to traverse the length of the tube, so that the eye receives the
light after it has passed through this length of 15 ft. of water, the
colour of the light is a strong blue. Water is blue in virtue of its
own molecular character, just as sulphate of copper is. Liquid oxygen,
prepared by the use of intense cold, is also transparent blue, and the
peculiar condensed form of oxygen known as "ozone" is, when liquefied,
of a darker or stronger blue than oxygen.
At one time (some thirty years ago) there was still some doubt as to
whether water was self-coloured blue, or whether its blue colour was
due to the action on light of excessively minute solid white particles
of chalk suspended in the water. Such fine suspended particles in some
cases act on the light which falls on to them so as to reflect the
blue rays. This occurs in certain natural objects which have a blue
colour. But these can be distinguished from transparent self-coloured
blue substances by the fact that whilst the light reflected from their
surface is blue, the light which is made to traverse them (when they
are held up to the light so that they come between one's eye and the
sun's rays) is brown. This is the case with very hot smoke, and can be
well seen when a cigar is smoked in the sunlight. The smoke which comes
off from the lighted end of the cigar is very hot, and its particles
are more minute than those of cooler smoke. The hot smoke shows a
bright blue colour when the sunlight falls on it and is reflected,
but when you look through the smoke-cloud at a surface reflecting the
sunlight, the cloud has a reddish-brown tint. As the smoke cools its
particles adhere to one another and form larger particles, and the
light reflected from the cloud is no longer blue but grey, and even
white. Thus the smoke which the smoker keeps for half a minute in his
mouth is cooled and condensed, and reflects white light–is, in fact, a
white cloud–when he puffs it out, and contrasts strongly with the blue
cloud coming off from the burning tobacco at the lighted end of the
cigar. The blue colour of the sky is held by many physicists (though
other views have been of late advanced) to be due to the same action
on the part of the very finest particles of watery vapour, which are
diffused through vast regions of our atmosphere above the condensed
white-looking clouds consisting of larger floating particles of water.
Vapours are given off by many liquids, and even by some solids, varying
in their production according to the heat applied in different cases.
They are gases, and quite transparent and invisible at the proper
temperature, like the atmospheric air. Thus water is always giving
off "water-vapour," which is quite invisible. When water is heated to
the boiling point it is rapidly converted into transparent invisible
vapour. Steam, as this vapour is called, is invisible, and we all
habitually make a misleading use of the word "steam" when we apply it
both to this and to the slightly cooled and condensed cloud which we
can see issuing from the spout of a kettle or from a railway engine.
It seems that the fault lies with the scientific writers, who have
applied the word "steam" to the invisible water vapour or gas before it
has condensed to form a cloud. The old English word "steam" certainly
means a visible cloudy emanation, and not a transparent invisible
gas. A cloud is not a vapour, but is produced by the coming together
or condensation of the minute invisible particles of a vapour to form
larger particles, which float and hang together, and reflect the light,
and thus are visible.
By the examination of other vapours or gases than that which is gaseous
water, namely, the vapours of bodies like chloroform and ether, it has
been shown that "cloud" forms in a vapour not merely in consequence of
the cooling of the vapour, but in consequence of the presence in the
air (or in the tube in which the vapour is enclosed for observation) of
very fine floating dust particles. They act as centres of attraction
and condensation for the vapour particles. When there are no dust
particles present clouds do not form readily in cooling vapours, or
only at lower temperatures, and in larger mass. Tyndall made some
beautiful experiments on this subject, obtaining clouds of great
tenuity in vapours enclosed in tubes, which reflected the most vivid
blue tints when illuminated by the electric arc-lamp. Later Aitken,
of Edinburgh, showed that the "fog" which forms in smoke-ridden towns
is due to the condensation of previously invisible watery vapour as
"cloud" around the solid floating particles of carbon of the smoke.
Aitken further used this property of solid floating particles, namely,
that they cause the formation of fog and cloud in vapours–to test the
question as to whether the excessively minute odoriferous particles
which affect our noses as "smell" are distinct solid floating particles
as often supposed, or are of the nature of gas and vapour. He admitted
strong perfumes such as musk into tubes containing watery vapour, at
such a temperature that the vapour was in a "critical" state–just ready
to condense and precipitate as "cloud." If he had admitted fine solid
particles such as a minute whiff of smoke, or some "dusty" air–the
cloud would have formed. But the admission of the perfume had no such
effect Therefore, he concluded that the odoriferous emanations used by
him are not distinct particles like those of smoke or dust, but are
gaseous.
The beautiful blue tint of the semi-transparent "white" of a boiled
plover's egg is due to a fine-particled cloud dispersed in the clear
albumen. London milk used to be "sky-blue" for a similar reason,
before the recent legislation against the adulteration of food. The
blue eyes of our fair-haired race and of young foxes are not due to
any "pigment"–that is to say, a separable self-coloured substance–but
to a fine cloud floating in a transparent medium which reflects blue
rays of light as blue smoke does. The iris of the eye can and often
does develop a pigment, but it is a brown one. When present in small
quantity it produces a green-coloured iris, the pale yellow-brown
being added to the blue cloud-caused colouring. When present in larger
quantity the same pigment gives us brown and so-called "black"
eyes. The blue colours in birds' feathers and insects' wings are
produced without blue pigment by special effects of reflection, and
where green is the colour it is often due to the addition of a small
quantity of yellow pigment to what would otherwise look blue: though
some caterpillars and grasshoppers have a real green pigment in their
skin. Flowers, on the other hand, have true soluble blue "pigments,"
and green ones too, notably that called leaf-green or chlorophyll.
The little green tree frog has no blue or green pigment in its skin;
only a yellow pigment. Sometimes rare specimens are found in which
the yellow pigment is absent altogether, and then the little frog is
turquoise-blue in colour. But there is no blue pigment in the skin;
only a finely-clouded translucent film overlying a dead-black deep
layer of the skin, and the result is that the frog is of a wonderful
pure blue. Sometimes the commoner large edible frog is found with a
similar absence of yellow pigment (I found some in a garden near Geneva
six years ago), and then all the parts of its skin which usually are
green show as brilliant blue.
It is at first difficult to believe that such fine, smoothly-spread
turquoise blue as that of the blue frog is due merely to a "reflection
effect," and that there is no blue pigment present which would show
as blue if light were transmitted through it, or could be separated
and dissolved in some medium. Yet this is undoubtedly the case. The
nearest experimental production of such a blue surface without blue
pigment is obtained by first varnishing a black board, and when the
varnish is nearly dry passing a sponge wetted with water over it.
Some of the varnish is precipitated from its solution in the spirit
(or it may be turpentine) as a fine cloud, and until the water has
evaporated it looks like blue paint, as the poet Goethe found when
cleaning a picture. It would be interesting to know more precisely the
precautions to be taken in order to get the blue colour in this way in
fullest degree.
It appears that when light is reflected from a cloud of fine colourless
particles so as to give a predominant blue colour, the light so
reflected is affected in that special way which physicists describe
as being "polarized." It is possible by the use of certain apparatus
(the polariscope) to distinguish polarized from non-polarized light,
so that it should be possible to decide (or at any rate to gain
evidence) whether blue water–a sheet of blue water–owes its colour to
fine particles suspended in it or to the self-colour of the water. An
admirable case for making this simple experiment is presented by the
great tanks–some 20 ft. cube–which are used by the water companies
which draw their water supply from the chalk, for the purpose of
precipitating the dissolved chalk–"Clarking" the water, as it is
called, after the inventor of the process–and so getting rid of its
excessive "hardness." Such tanks are to be seen by the side of the
railway near Caterham. The water in these tanks is of such a brilliant
turquoise blue that many people suppose that copper has been added
to the water to free it from microbes! Such, at any rate, was the
conviction expressed by a friend in conversation with me only a few
weeks ago. The water in these tanks, when seen from the railway, looks
like a magnificent blue dye, and a very important point for those (not
a few) who believe that the blue colour of seas and lakes is due to the
reflection of the blue colour of the sky overhead is that the water
in the tanks looks just as blue when the sky is overcast with cloud
as when there is blue sky. The blue colour of water has, as a rule,
nothing to do with the reflection of the sky, though it is the fact
that a shallow film of water may at a certain angle reflect the sky to
our eyes, just as a mirror may. The effect is quite unlike that due to
light passing through deep water when reflected from below it. If we
examine the tanks in question we find that they have been filled with
water pumped from the chalk, and that then lime has been added to the
water in order to combine with the carbonic acid dissolved in it and
form chalk or carbonate of lime–which is insoluble in pure water and
falls as an excessively fine white powder to the bottom of the tank.
But the important fact is that water having carbonic acid dissolved
in it can dissolve carbonate of lime or chalk to a certain amount:
and this water pumped from the chalk, having carbonic acid naturally
dissolved in it, has consequently also dissolved a quantity of chalk.
It is this which gives the chalk-spring water the objectionable
quality of "hardness." When lime is added to the chalk-spring water
as pumped into the tanks, the carbonic acid in it is taken up by the
lime, and the chalk previously dissolved by the carbonic-acid-holding
water is, so to speak, "undissolved," and thrown down as a very fine
white powder, together with the chalk newly formed by the union of the
lime and the carbonic acid. These large tanks are used to allow the
fine powder of chalk to settle down and leave the water clear. The
brilliantly white chalk sediment accumulates not only on the floor
of the tank, but on its sides. Any light which falls on the tank is
refracted and reflected from side to floor and from floor to side, and
eventually emerges from the tank, a great deal of it having traversed
the 20 ft. breadth and depth many times. Most of its red, yellow, and
green rays are quenched by the many feet of blue water through which
it has passed, and it issues as predominantly blue. This is largely
due to the fine reflecting surface furnished by the "white-washed"
or chalk-coated floor and sides and the great purity of the white
reflecting material–no yellow or brown matter being present to give a
greenish tinge to the result I remember being taken to see "Clark's
process" in use, and the splendid blue colour of the water in the
"softening" tanks at Plumstead, when the process was first used by the
North Kent Water Company, sixty-four years ago.
It is, I think, still a possible question as to whether the fine
floating particles of precipitating chalk act in any way as a
"cloud"–in short, as the blue clouds of smoke, egg-white, milk, and
varnish. There is no evidence that they do, but no one, so far as I
know, has ever taken the trouble to settle the question. It could be
done by examining the blue light from the tanks with a polariscope,
and also by sinking a black tarpaulin into the tank to cover the white
floor and hanging others at the sides. Then if the blue colour were due
to light reflected from the white floor and sides traversing repeatedly
the clear self-coloured blue water, the blue colour should no longer
be visible, for the reflecting surfaces would be covered by the black
tarpaulin and little light sent up through the water. But if it were
due to a cloud of greatest delicacy in the water–like fine smoke
reflecting the blue light rather than the other rays–then the colour
should be as intense or more intense when the black background is
introduced. I am surprised that some inquirer, younger and more active
than I am, does not put the matter to the test of experiment.
On the whole, practically all the facts which we know about "blue
water" are consistent with the blue self-colour of water, and not with
that of a "blue cloud" in the water. Now that we have porcelain baths
of the purest white and of large size, one may often see the strong
blue colour of water of great purity in the bath, especially where
waves or ripples send to our eyes those rays of light which have taken
a more or less horizontal course from side to side of the bath, and
have thus been through a large thickness of the pale-coloured fluid.
Great masses of clear ice, such as one may study in glaciers, are
blue; the "crevasses" which transmit light which has passed through
a considerable thickness of ice (as, for instance, in an ice cave),
are deep blue; there is no question of a reflection from suspended
particles. The green colour which some glaciers show at a little
distance is due to the yellow rust–iron oxide–blown on to the surface
of the ice and dissolved. Many glaciers or parts of glaciers are quite
free from it, and of a splendid indigo blue in their deeper fissures.
So, too, as to the sea and lakes. The Blue Grotto or Cavern of the
island of Capri, near Naples, is a case in point. All the light which
enters it comes through the sea-water and is blue. I was taken to it
in a boat rowed by two men. As the boat enters the low mouth of the
cavern you have to bend down to avoid knocking your head against the
rock. Then you find yourself floating in a vast and lofty chamber the
white rocky floor of which is some twenty feet below the surface of the
clear water. No light enters the cavern by the low part of the entrance
above water. Below the surface it widens and the strong Southern sun
shines through the clear water and its light is reflected up into the
cave from the bottom. It is blue, and everything in the cave above as
well as below the water is suffused with a blue glow–a truly wonderful
and fascinating spectacle. In order to get the best effect you must
choose an hour when the sun is in a favourable position. Where there
is a white bottom at a depth of fifty or a hundred feet, the sea has
a fine ultra-marine colour, so long as it is clear. It is often made
green by yellow-coloured impurities, either fine iron-stained sediment
or by minute living things in the water. The colour of the water of
either sea or lakes, when it is clear and overlying great depths (200
fathoms and more), tends to be dark indigo owing to the deficiency of
reflected light. But there are enough white particles as a rule to send
some of the light, which penetrates the water, upwards again. Even the
great ocean has a dark purplish-blue colour, but never the bright blue
of clear water in shallow seas with light-coloured or white bottom.
One of the most beautiful exhibitions of the colour of clear water in
various thicknesses which I know, is at the entrance of the Rhone into
the Lake of Geneva. The thick pale-coloured brownish-white sediment
of the river shoots out for a quarter of a mile or more into the dark
blue waters of the deep lake, and on a bright sunny day as it subsides
reflects the light upwards from different depths through the clear
water. Where it has sunk but little the colour is green, owing to the
influence of the yellow mud. Farther on it is ultra-marine blue, and
then, where it has sunk deeper, we get full indigo tints. The movement
of the water and its churning up by the steamers' paddles add to the
variety of effects, since the foam of air-bubbles submerged throws
up the light through the water. It is not possible to doubt as one
watches the admixture of the river and the lake, and the eddies and
hanging walls of sediment, that one is floating over a vast depth
of magnificent blue self-coloured fluid which is traversed by the
sunlight in ways and degrees varying according to its depth and the
volume of the pale mud of the in-rushing Rhone and the abundance of
fine air-bubbles "churned" into the water by the paddle-wheels of the
steamer.