504 THE POPULAR SCIENCE MONTHLY. 



We possess three metals distinguished for their great density iridi- 

 um, 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. Be- 

 cause 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, ac- 

 cording 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 wa- 

 ter. The specific weight of Saturn is 0*73 ; of Uranus, 0*84 ; conse- 

 quently, 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 cen- 

 ter, 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- 



