and cities could be thus destroyed, with the certainty that the airship could come to no harm as the missiles could be hurled from a vast height.' The extravagance of Lana's design must not be allowed to rob him of the credit of being, in some sense, the inventor of the balloon. A balloon filled with gas, and lighter than air, was in his day inconceivable; the composition of the atmosphere was unknown, and the chemistry of gases was not understood. But he had followed the physical investigations of the seventeenth century, and was well acquainted with Torricelli's demonstration of the weight of the atmosphere. The only practical way for him to make a vessel lighter than air was to empty it of the air within it, and Torricelli's invention of the barometer seemed to bring such a device within reach. The common pump begat the barometer; the barometer begat the balloon. But the enormous pressure of the atmosphere on a vessel encasing a vacuum, though Lana had triumphed over it in argument, could not be so easily dealt with in practice. The success of the balloon was delayed until, by the discovery and production of a gas lighter than air, a frail and thin envelope could be supported against the pressure from without by an equal pressure from within. For ballooning what was chiefly necessary was a thorough knowledge of gases and of the means of producing them. The older chemistry, or alchemy, devoted all its attention, for centuries, to the precious metals, and knew nothing of gas. Medical chemistry, which succeeded it, was concerned chiefly with the curative properties of various chemical preparations. When Robert Boyle, and the investigators who came after him, put aside this age-long preoccupation with wealth and healing, and set themselves to determine, by observation and experiment, the nature of common substances, and the possibility of resolving them into simpler elements, modern chemistry began. Four states of matter, namely, earth, air, fire, and water, were recognized by the older chemists, and were by them called elements; it was the work of the eighteenth century to investigate these, and especially to separate the constituents of air and of water. In 1774 Joseph Priestley discovered oxygen. In 1782 Henry Cavendish showed that hydrogen, when burnt, produces water. At a much earlier date hydrogen had been produced by the action of acid on metals, and had been found to be many times lighter than air. Dr. Joseph Black, professor of chemistry in the University of Edinburgh, was the first to suggest, in 1767, that a balloon inflated with hydrogen would rise in the air; and the experiment was successfully tried with soap-bubbles by Tiberius Cavallo, in the year 1782.