But can we not go back one step farther still in the progressive stages of creative energy? Whence came these powerful agencies by means of which all those distant regions became peopled with suns and worlds? The great source of all is to be found alone in space,--the so-called "empty space." But it is far from empty; all through it are diffused the attenuated vapors which, condensed, constitute our suns and planets, and all that is, or ever shall be, gaseous vapors, which are held poised, with their opposite tensions of cohesion and expansion, like the Prince Rupert drops which glass-blowers make for toys,--a little bulb of glass, chilled as it falls, molten, in a vessel of water. From one extremity projects a long, crooked stem, scarcely thicker at the end than a horse-hair, spun out from the molten glass as it hung from the glass-blower's rod. The bulbous body is as large, perhaps, as a nut; you can beat it with a hammer and it will not break; it is the hardest in structure of all glass. Now, wrap this bulb up in a thick handkerchief, or you may be injured; hold it firmly, and break off the very tiniest tip of the long stem three, four, or even six inches from the bulb. There is a sudden shock; open your handkerchief, and lo! instead of the solid bulb, there is only a loose mass of white powder. If you put the bulb in a heavy glass vessel full of water and break off the tip of the tail, it will shatter the vessel into fragments. What is the explanation?--it is, of course, well known--simply that the molecules of glass were instantly arrested in their motion of adjustment as the glass was suddenly chilled by the water, and the molecular motion of shrinkage was arrested, leaving the individual molecules under a tremendous strain of position in their endeavor to reach their true places. They are rigidly fixed in this position of unstable equilibrium, one balancing the other; but let a single molecule be displaced,--a fragment so tiny that the eye can scarcely see it,--and the molecules, thus thrown out of mutual support against each other, must now rearrange themselves from the ruptured rigid mass, and, like a row of stood-up bricks, each of which thrusts the other forward, with a sudden explosive force the molecules assume their true position of stable equilibrium, but it is at the cost of the whole structure. To this same cause we owe the explosive force of our gunpowder, nitroglycerin, and all explosives; the molecules are held in unstable equilibrium, and the tension once relieved at a single point, be it ever so infinitesimal, the molecules of the whole mass rearrange themselves with explosive energy. Strange that so harmless a substance as glycerin, by the mere replacement of an atom of nitrogen gas, should develop the energy of dynamite under a trifling molecular shock.