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.
CHAPTER IX
AMBER
AMBER is not unfrequently picked up among the pebbles of the East Coast. I once picked up a piece on the beach at Felixstowe as big as a turkey's egg, thinking it was an ordinary flint-pebble and intending to throw it into the sea, when my attention was arrested by its extraordinary lightness, and I found that I had got hold of an unusually large lump of amber. There is a locality where amber occurs in considerable quantity. It is a long way off—namely, the promontory called Samland near Königsberg on the Prussian shore of the Baltic. There it occurs with fossil wood and leaves in strata of early Tertiary age, deposited a little later than our "London clay." It used to be merely picked up on the shore there until recent times, when "mining" for it was started. From this region (the Baltic coast of Prussia) amber was carried by the earliest traders in prehistoric times to various parts of Europe. Their journeyings can be traced by the discovery of amber beads in connexion with interments and dwelling-places along what are called "amber routes" radiating from the amber coast of Prussia. To reach the East Coast of England the bits of amber would have to be carried by submarine currents. Amber travels faster and farther than ordinary stones, on account of its lightness. What has been held to be amber is found, also embedded in ancient Tertiary strata, in small quantity in France, in Sicily, in Burma, and in green sand (below the chalk) in the United States. The Sicilian amber (called "Simetite") was not known to the ancients: it is remarkable for being "fluorescent," as is also some recently discovered in Southern Mexico. But it is possible that chemically these substances are not quite the same as true amber. Amber is a fossil resin or gum, similar to that exuded by many living trees, such as gum-copal. It has been used as an ornament from prehistoric times onwards, and was greatly valued by the Egyptians, Greeks, and Romans, and by our Anglo-Saxon ancestors, not only for decorative purposes, but as a "charm," it being supposed to possess certain magical properties.
Amber (it is generally believed) comes slowly drifting along the sea bottom to the Suffolk shore from the Baltic. Lumps as big as one's fist are sometimes picked up here. The largest pieces on record found on the Baltic shore, or dug out of the mines there, are from 12 to 18 lb. in weight, and valued at £1000. A party sent by the Emperor Nero brought back 13,000 lb. of amber from the Baltic shores to Rome. The bottom currents of seas and oceans, such as those which possibly bring amber to our shores, are strangely disposed. The Seigneur of Sark some fifty years ago was shipwrecked in his yacht near the island of Guernsey; he lost, among other things, a well-fastened, strongly-made chest, containing silver plate. It was found a year later in deep water off the coast of Norway and restored to him! In the really deep sea, over 1000 fathoms down, there are well-marked broad currents which may be described as rivers of very cold water (only four degrees or so above freezing-point). They flow along the deep sea bottom and are sharply marked off from the warmer waters above and to the side. Their inhabitants are different from those of the warmer water. They are due to the melting of the polar ice, the cold water so formed sinking at once owing to its greater density below the warmer water of the surface currents. These deep currents originate in both the Arctic and Antarctic regions, and the determination of their force and direction, as well as of those of other ocean currents, both deep and superficial, such as the warm "Gulf Stream," which starts from the Gulf of Mexico, and the great equatorial currents, is a matter of constant study and observation, in which surveying ships and skilled observers have been employed.
Amber has not only been valued for its beauty of colour—yellow, flame-colour, and even deep red and sometimes blue—for its transparency, its lightness, and the ease with which it can be carved, but also on account of certain magical properties attributed to it. Pliny, the great Roman naturalist of the first century A.D., states that a necklace of amber beads protects the wearer against secret poisoning, sorcery, and the evil eye. It is first mentioned by Homer, and beads of it were worn by prehistoric man. Six hundred years B.C., a Greek observer (Thales) relates that amber when rubbed has the power of attracting light bodies. That observation is the starting-point of our knowledge of electricity, a name derived from the Greek word for amber, "electron." In Latin, amber is called "succinum." By heating in oil or a sand-bath, amber can be melted, and the softened pieces squeezed together to form larger masses. It can also be artificially stained, and cloudy specimens are rendered transparent by heating in an oil-bath.