We possess three metals distinguished for their great density—iridium, platinum, and gold. If we assume the weight of water as unity, the gravity of these metals—is iridium, 22·23; platinum, 21·50; gold, 19·25. With the exception of iridium, rare as well as remarkable, gold and platinum weigh more than any other metal: for instance, lead has a specific gravity of 11·35; silver, 10·47; copper, 8·80; and iron, only 7·84. The question occurs next, whether the scarcity of gold stands in any connection with its gravity.
Since the earth originally passed from a gaseous into a fluid state, the heaviest components must, in its fluid condition, have tended toward the center. If it he true that our entire planetary system developed from an immense nebula, it follows that the planets nearest to the center must be the heaviest.
"Their great specific gravity," says Professor Petzholt, speaking of metals, "is the reason that they can so rarely be seen on the surface of the earth; the largest masses are to be found within the interior, in a molten condition, and are there protected against the cupidity of man."
Further observations have confirmed the truth of this view. Because spectral analysis reveals no gold in the sun, we must accept the fact that it lies hidden in its interior, and that it is covered by lighter bodies, which, in a gaseous state, form the photosphere of the sun. Hydrogen, the lightest of all gases, constitutes the chief component of this photosphere. Now, the planets are divided into two groups, according to their weight: the heavy, which lie within, and the light, which lie beyond, the circle of the asteroids. Mercury, the nearest to the sun, is seven times as heavy as water; Venus, the Earth, and Mars are five times as heavy; while Jupiter barely attains the weight of water. The specific weight of Saturn is 0·73; of Uranus, 0·84; consequently, they are lighter than water. The density of Neptune, not yet exactly determined, is at any rate very small. We therefore find in our planetary system that the densest bodies are nearest to the center, and this leads us to the presumption that the same law governs on each individual planet, and that, therefore, the heavy gold must be found nearest to the center.
This leads to the question, Whence the gold and platinum to be found upon the surface of the earth? These two heavy metals are only found in places where volcanic rocks have penetrated through earlier formations, and the granite has split up. Platinum, only found in excess in the Ural Mountains, is obtained from rocks that have come up from below; gold is found only in quartz-veins. These veins have been formed in the following manner: As a natural consequence of the contraction of the earth's crust, internal revolutions, and volcanic eruptions, crevices opened, which were partly filled by hot springs, partly by eruptions with quartz. Rich deposits are often found in these crevices, called by the American miners "pockets," or "bonan-
