QUICKSANDS AND FIRE-STONES
THERE are curious facts about sand which can be studied on the
seashore. There are the "quicksands," mixtures of sand and water, which
sometimes engulf pedestrians and horsemen at low tide, not only at
the Mont St. Michel, on the Normandy coast, but at many spots on the
English, Welsh, and Scotch coasts. Small and harmless quicksands are
often formed where the sand is not firmly "bedded" by the receding sea,
and the sea-water does not drain off, but forms a sort of sand-bog.
Then one may also study the polishing and eroding effect of dry blown
sand, which gives a "sand-glaze" to flints, and in "sand-deserts" often
wears away great rocks. The natural polishing of flints and other
hard bodies by fine sand carried over them for months and years in
succession by a stream of water, is also a matter of great interest,
about which archæologists want further information.
A very interesting fact about the ordinary sand of the seashore is
that two pints of dry sand and half a pint of water when mixed do not
make two pints and a half, but less than that quantity. If you fill a
child's pail with dry sand from above the tide-mark, and then pour on
to it some water, the mass of sand actually shrinks. The reason is that
when the sand is dry there is air between its particles, but when the
sand-particles are wetted they adhere closely to each other; the air is
driven out, and the water does not exactly take an equivalent space,
but occupies less room than the air did, owing to the close clinging
together of the wet particles. If you add a little water to some dry
sand under the microscope, you will see the sand-particles move and
cling closely to one another. "Capillary attraction"--the ascent of
liquid in very fine tubes or spaces--is a result of the same sort of
adhesive action. If you walk on the firm, damp sand exposed at low tide
on many parts of the seashore when it is just free from water on the
surface, you will see that when you put your foot down the sand becomes
suddenly pale for some seven inches or so all round your foot. The
reason is that the water has left the pale-looking sand (dry sand looks
paler than wet sand), and has gone into the sand under your foot, which
is being squeezed by your weight. The water passing into that squeezed
sand enables its particles to sit tighter or closer together, and so
to yield to the pressure caused by your weight. You actually squeeze
water "into" the sand, instead of squeezing water "out" of it, as is
usually the case when you squeeze part of a wet substance--say a cloth
or a sponge. When you lift your foot up, you find that your footmark is
covered with water--the water you had drawn to that particular spot by
squeezing it. It separates as soon as the pressure is removed.
Quartz and quartzite pebbles occur on the South as well as the East
Coast. They are sometimes called "fire-stones," because they can
be made to produce flashes of flame. If you take a couple of these
pebbles, each about as big as the bowl of a dessert-spoon (a couple of
flint pebbles will serve, but not so well), and holding one in each
hand in a dark room, or at night, scrape one with the other very
firmly, you will produce a flash of light of an orange or reddish
colour. And at the same time you will notice a very peculiar smell,
rather agreeable than otherwise, like that of burning vegetable matter.
It would seem that the rubbing together of the stones produces a fine
powder of some of the siliceous substance of the stone and at the
same time a very high temperature, which sets the powder aflame. I
had the idea at one time, based on the curious smell given out by the
flashing pebbles, that perhaps it was a thin coating of vegetable or
other organic matter derived from the sea-water which burns when the
stones are thus rubbed together; but I found on chemically cleaning
my pebbles, first with strong acid and then with alkali, that the
flame and the smell were produced just as well by these chemically
clean stones as by those taken from the beach. The flame produced
by the rubbing of the two stones seemed then to be like the sparks
obtained by strike-a-lights of flint and steel, or the prehistoric
flint and pyrites. Now, however, a new fact demands consideration. The
supposition that the powdered silica formed, when one rubs the two
pebbles together, is actually "burnt," that is to say, combined with
the oxygen of the air by the great heat of the friction, is rendered
unlikely by the fact that if you perform the rubbing operation in a
basin of water with the stones submerged, the flash is produced as
easily as in the air. My attention was drawn to this fact by a letter
from the well-known naturalist the Rev. Reginald Gatty. I at once tried
the experiment and found the fact to be as my correspondent stated. Not
only so, but the smell was produced as well as the flash.
With the desire to get further light on the subject, I consulted
the great experimental physicist, my friend Sir James Dewar, in his
laboratory at the Royal Institution. He told me that the late Professor
Tyndal used to exhibit the production of flame by the friction of two
pieces of quartz in his lectures on heat, but made use of a very large
and rough crystal of quartz (rock-crystal) and rubbed its rough surface
with another large crystal. Tyndal's note on the subject in his lecture
programme was as follows (Juvenile Lectures on Heat, 1877-78): "When
very hard substances are rubbed together light is produced as well as
heat." Sir James Dewar kindly showed me the crystals used by Tyndal,
the larger was 16 inches long and 4 or 5 inches broad. We repeated the
experiment in the darkened lecture room, and obtained splendid flashes.
The same smell is produced when rock-crystal is used as when flint or
quartz pebbles are rubbed together. All three are the same chemical
body, namely, silica (oxide of silicon). We also found that when the
crystals were bathed with water or (this is a new fact) with absolute
alcohol, the same flashing was produced by the friction of one against
the other.
Later, with the kind assistance of Mr. Herbert Smith, of the mineral
department of the Natural History Museum, I examined, with a
spectroscope, the flash given by two quartzite pebbles when rubbed
together. No distinctive lines or bands were seen; only a "continuous"
spectrum, showing that the temperature produced was not high enough to
volatilize the silicon. I also examined some pebbles of another very
hard substance--nearly as hard as silica (rock-crystal, quartz, and
flint). This was what is called "corundum," the massive form of "emery
powder" (oxide of aluminium). By grinding two of these corundum pebbles
with very great pressure one against the other (using much greater
pressure than is needful in the case of quartz), I obtained flashes
of light. It was not known previously that any pebbles except those
of silica would give flashes of light when rubbed together. A smell
resembling that given out by rubbed quartz, but fainter, was observed.
Those are the facts--new to me and to many others--about this curious
subject. The flashing under water is a very remarkable thing. I cannot
say that I am yet satisfied as to the nature of the flash. A simple
explanation of the result obtained, when two dry pebbles are rubbed
together in the air, is that crushed particles of the quartz or of
the corundum are heated by the heavy friction to the glowing point.
But this does not accord with the fact that submergence in a liquid
does not interfere with the flashing. The rise of temperature would
certainly be checked by the liquid. And the curious smell produced is
in no way explained.
The breaking of crystals is in many instances known to produce a
flash of light. Thus a lump of loaf sugar broken in the dark gives a
faint flash of blue light, as anyone can see for himself immediately
on reading this. White arsenic crystals also, when broken by shaking
the liquid in which they have formed, give out flashes of light. Some
rare specimens of diamond, when rubbed in the dark with a chamois
leather, glow brightly. The well-known mineral called Derbyshire spar,
"Blue John," or fluoride of calcium, when heated to a point much below
that of a red-hot iron, "crackles" and glows briefly with a greenish
light. The crystals of phosphate of lime, called apatite, and a number
of other crystals have this property. But there is no record of any
peculiar smell accompanying the flashes of light. It is still a matter
open to investigation as to whether the flashing of pieces of quartz
and rock-crystal when rubbed together with heavy pressure is of the
nature of the flashing of the heated crystals of other minerals, or
whether there is any chemical action set up by the friction--an action
which is certainly suggested by the very peculiar smell produced.
Since the flashing can be produced under water and other liquids, it
should be easy to obtain some evidence as to the chemical nature of the
flame--whether acid or alkaline, whether capable of acting on this or
that reagent dissolved in the water, and whether setting free any gas
of one kind or another.
Any one of my readers who chooses can produce the wonderful
orange-coloured flame by rubbing two quartz or flint pebbles together
in the dark, and can have the further gratification of producing with
the utmost ease the mysterious and weird phenomenon of a flame under
water, and may, perhaps, by further experiment, explain satisfactorily
this unsolved marvel which has haunted some of us since childhood.