The two greatest revolutions which have taken place in the annals of the species, and which have for ever left their traces on the fortunes of mankind, have arisen from the successive diminution and increase in the supply of the precious metals for the use of the world. There can be no doubt that the decline and fall of the Roman empire--so long and falsely ascribed to its latter extension, plebeian slavery, and patrician corruption--was in reality mainly owing to the failure in the mines of Spain and Greece, from which the precious metals in ancient times were chiefly obtained, joined to the unrestricted importation of grain from Egypt and Libya, which ruined the profit of the harvests and destroyed the agriculture of Italy and Greece, at once paralysing industry, and rendering taxes overwhelming.[1] We know now to what the failure of these mines, attended with such portentous results, was owing. It was to the exhaustion of the auriferous veins in Spain and Greece near the surface, from long-continued working, and the extreme _hardness of the rock_ in which they were imbedded farther down, which seems to be a general law of nature all over the world,[2] and which rendered working them, to any considerable depth, no longer a source of profit. On the other hand, the prodigious start which Europe took during the sixteenth and seventeenth centuries, and which has implanted the European race for ever in the new hemisphere, is well known to have been mainly owing to the discovery of the mines of Mexico and Peru, and the continual rise of prices during nearly two centuries, which took place all over the world, from the constant and increasing influx of the precious metals drawn out of their rich strata.