Fiction has employed itself with the utmost license in giving glowing pictures of treasures hidden in the earth’s recesses. The caverns of Chilminar, the cave of Aladdin, the abodes of the spirits of the Hartz, and the dwellings of the fairies of England, are gem-bespangled and gold-glistening vaults, to which man has never reached. The pictures are pleasing; but although they have the elements of poetry in them, and delight the young mind, they want the sterling character of scientific truth; and the wonderful researches of the plodding mineralogist have developed more beauty in the caverns of the dark rock than ever fancy painted in her happiest moments. In all probability the action of the sun’s rays upon the earth’s surface, producing a constantly varying difference of temperature, and also the temperature which has been observed as existing at great depths, give rise to thermo-electrical currents, which may play an important part in the results thus briefly described. In connection with these great natural operations, explaining them, and being also, to some extent, explained by them, we have the very beautiful application of electricity to the deposition of metals, called the Electrotype. Applying the views we have adopted to this beautiful discovery,[165] the whole process by which these metallic deposits are produced will be yet more clearly understood. By the agency of the electric fluid, liberated in the galvanic battery, a disturbance of the electricity of the solution of copper, silver, or gold, is produced, and the metal is deposited; but, instead of allowing the acid in combination to escape, it has presented to it some of the same metal as that revived, and, consequently, it combines with it, and this compound, being dissolved, maintains the strength of the solution.[166] A system of revival, or decomposition, is carried on at one pole, and one of abrasion, or more correctly speaking, of composition and solution, at the other. By taking advantage of this very extraordinary power of electricity, we now form vessels for ornament or use, we gild or silver all kinds of utensils, and give the imperishability of metal to the most delicate productions of nature--her fruits, her flowers, and her insects;--and over the finest labours of the loom we may throw coatings of gold or silver to add to their elegance and durability. Nor need we employ the somewhat complex arrangement of the battery: we may take the steel magnet, and, by mechanically disturbing the electricity it contains, we can produce a current through copper wires, which may be used, and is extensively employed, for gilding and silvering.[167] The earth itself may be made the battery, and, by connecting wires with its mineral deposits, currents of electricity have been secured, and used for the production of electrotype deposit.[168]