Jan Baptista van Helmont, a Flemish physician (1577-1644), was the first to apply the term, _gas_ to the elastic fluids which resemble air in physical properties. Robert Boyle, the celebrated Irish scholar and scientist, and improver of the air pump, and Edwin Mariotte, the French physicist who was first to show that a feather and a coin will drop the same distance at the same time in a reservoir exhausted of air, were the independent discoverers of Boyle’s and Mariotte’s law of gases(1650-1676). This was that at any given temperature of a gas which is at rest its volume varies inversely with the pressure put upon it. It follows from this law that the density and tension, and therefore the expansive force of a gas, are proportional to the compressing force to which it is subjected. It is said that Abbé Hauteville, the son of a baker of Orleans, about 1678 proposed to raise water by a powder motor; and that in 1682 he described a machine based on the principle of the circulation of the blood, produced by the alternate expansion and contraction of the heart. The production of heat by concentrating the rays of the sun, and for burning objects had been known from the time of Archimedes, and been repeated from time to time. Thus stood this art at the close of the 17th century, and thus it remained until near the close of the 18th. In England Murdock, the Cornish Steam Engineer, was the first to make and use coal gas for illuminating purposes, which he did in 1792 and 1798. Its utilisation for other practical purposes was then suggested. Gas engines as motive powers were first described in the English patent to John Barber, in 1791, and then in one issued to Robert Street in 1794. Barber proposed to introduce a stream of carbonated hydrogen gas through one port, and a quantity of air at another, and explode them against the piston. Street proposed to drive up the piston by the expansive force of a heated gas, and anticipated many modern ideas. Phillipe Lebon, a French engineer, in 1799 and in 1801 anticipated in a theoretical way many ideas since successfully reduced to practice. He proposed to use coal gas to drive a piston, which in turn should move the shaft that worked the pumps which forced in the gas and air, and thus make the machine double-acting; to introduce a charge of inflammable gas mixed with sufficient air to ignite it; to compress the air and gas before they entered the motor cylinder; to introduce the charge alternately on each side of the piston; and he also suggested the use of the electric spark to fire the mixture. But Lebon was assassinated and did not live to work out his ideas.