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gravity, and unalterability by air and water, are qualities which will probably lead to an extensive employment when a cheaper method of production shall have been discovered; but even now it has found a highly interesting use. It is so easily inflammable that a wire of considerable thickness can be ignited in the flame of a candle, and the light evolved by the combustion is of almost solar intensity. In lighthouses it serves to guide the mariner in his course; it lights up the obscurest recesses of stalactital caverns, and with its assistance the photographer no longer depends upon the sun, and reveals to us the hidden paintings and sculptures of rocktombs and temples as distinctly as if they were exposed to the light of day.

Sodium, the metallic basis of soda, was discovered by Sir Humphry Davy in 1807. It is lighter than water, and white and lustrous as silver; but exposure to air almost immediately converts it into soda. Thus it can never become directly useful, like aluminium or magnesium; but being indispensable for reducing the ores of these two metals, it renders important indirect services, and is consequently produced in considerable quantities.

Palladium, one of the hardest and heaviest of metals, is of a steel grey colour, passing into silver white. Its alloy with silver, which has the valuable property of not tarnishing in air, is eminently fitted for the manufacture of delicate scientific instruments. The Wollaston medal, given by the Geological Society, is, in honour of its discoverer, made of palladium, which is considerably dearer than gold.

In 1804, the same eminent philosopher discovered another metal in native platina, to which he gave the name of Rhodium. Mixed with steel in the proportion of one to fifty, rhodium produces an excellent metal for making the sharpest cutting instruments, and a mixture of equal parts of rhodium and steel makes the best telescopic mirrors, as it is not liable to be tarnished. It is also employed for making the unalterable nibs of the so-called rhodium pens.

Thallium, though one of the newest metals, as it was discovered by Mr. Crookes as recently as 1861, already bids fair to render some important services. It imparts to optical glasses a considerable density and dispersive power, and

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should no other use be found for it, this alone would render it a valuable acquisition.

Such is the brief history of those new metals which have already found a useful employment in the industrial arts. It throws a vivid light upon the rapid progress of modern chemistry, for the very existence of most of them was undreamt of at the beginning of the present century, and their discovery could be attained only by an amount of analytical knowledge beyond the scope of any previous age. On witnessing these triumphs of science we may well ask where they will end, and when the goal will be reached beyond which it will be impossible for the human intellect to penetrate ?

CHAPTER XXXII.

COAL.

The Age of Coal-Plants of the Carboniferous Age-Hugh Miller's Description of a Coal Forest-Vast Time required for the Formation of the Coal-fields-Derangements and Dislocations-Faults-Their Disadvantages and Advantages-Bituminous Coals-Anthracites-Our Black Diamonds-Advantageous Position of our Coal Mines-The South Welsh Coal-field-Great Central and Manchester Coal-fields-The Whitehaven Basin and the Dudley Area-Newcastle and Durham Coal-fields-Costly Winnings-A Ball in a Coal-pit-Submarine Coal Mines -Newcastle View from Tynemouth Priory-Hewers-Cutting MachinesPutters-Onsetters — Shifters-Trapper Boys-George Stephenson-Rise of Coal Production-Probable Duration of our Supply-Prussian Coal MinesBelgian-Coal Mines in various other countries-Maunch Chunck.

HE history of the primitive races of mankind, as far as we

their existence, shows us that an age of stone was followed by one of bronze, which in its turn was succeeded by one of iron. The Golden Age has probably never existed but in the fancy of poets who sought in the land of dreams a compensation for the deficiencies of the real world; and there can be no doubt that, despite California and Australia, our own times are as far from realising the pleasing vision as any before them.

But a title to which they have a better claim is founded upon the vast use of the mineral fuel without which the glorious inventions of Watt and Stephenson would have been comparatively vain; and whoever has attentively examined the foundations of our industry, our commerce, our wealth, and our civilisation will hardly deny that we live in what may justly be termed the Age of Coal.

This mineral, the importance of which in the political economy of the leading nations of the globe can hardly be overrated, is also one of surpassing interest in a geological

IMPORTANCE OF COAL.

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point of view, for the history of its formation is one of the great marvels of the subterranean world.

The plants whose growth and decay originally furnished the materials of which our black coal* is composed, flourished in that far distant period when as yet no bird or mammalian quadruped had made its appearance, when even the gigantic Ichthyosaurus was not yet born, and the progress of organic life had not advanced beyond the creation of some uncouth reptiles or strangely formed fishes. From the vast space of time which separates us from the carboniferous age, it may

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easily be imagined that the state of the vegetable world was then extremely different from that now prevailing. The vegetable remains which constitute coal have generally been so transformed as to afford no trace of their original texture; yet the distinct plants found here and there preserved in the mass, and which amount to about five hundred species, plainly bear the character of a swampy vegetation, and show that they must have grown in submerged, or at least extremely humid, situations.

* Lignite, or brown coal, is of more modern origin.

They

consist chiefly of ferns, of Lepidodendra, allied to the clubmosses of the present day, of a few coniferous trees, the

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woody structure of some of them showing that they were

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