GASES, GENERAL PROPERTIES or. The term gas—which is from the same root as ghost, Ger. geist, breath, spirit—was employed by the older chemists to designate any kind of air or vapor. all liehnont was the first chemist who limited the term gas to such elastic fluids as had not been rendered liquid or solid by a reduction of tempera ture. In common language a distinction is made between gases and vapors. Gases are understood to be invariably Oriform at ordinary temperatures and atmospheric pressures, while vapors under these conditions are solid or liquid, and only assume a vaporous or apparently gaseous form at relatively high temperatures. Thus oxygen, hydrogen, nitrogen, chlorine, etc., are considered true gases; while water, sulphur, iodine, etc., when heated to certain definite points, become transformed into vapors. There is, however, no distinction between gases and vapors in a theoretic point of view.
'The kinetic theory of gases, first put forth by Daniel Bernouilli. is to the effect that they are formed of material particles, free in space, and actuated by very rapid rectilin ear movements, and that the tension of plastic fluids results from the shock of their particles against the sides of the containing vessels. This theory has been recently reviver{ and developed chiefly by Clausius and Clerk Maxwell. Their perfect elasticity is one of the most important physical peculiarities of gases. Within the limits of all ordinary experiments it is generally true that " the volume of a gaseous body is inversely as the compressing force." See MAPJOTTE'S LAW.
In consequence of their extreme elasticity, gases exhibit an entire absence of colic lion among their particles, and in this respect they differ essentially from liquids. A vessel may be filled either partially or completely with a liquid, and this liquid will have a definite level surface or limit. With gases, it is otherwise; they always perfectly fill the vessel that contains them, however irregular its form. Instead of cohesion, there is . . • • a mutual repulsion among their particles, which have a continual tendency to recede further from each other, and thus exert a pressure in an outward direction upon the Sides of the vessel in which the gas is inclosed. • This outward pressure is greater or less according as the elasticity of the gas is increased or diminished.
Dalton long ago remarked that "there can scarcely be a &MIA entertained respecting the reducibility of all elastic fluids, of whatever kind, into liquids; and we ought not to despair of effecting it at low temperatures and by strong pressure exerted upon the unmixed This prediction has been completely fulfilled. It occurred to Fara
day, Who led the van in these investigations, that the most probable mode of obtaining gases (or rather what, under ordinary circumstances, would be gases) in the liquid state, would be to generate them under strong pressure. When thus produced in strong bent glass tubes, they continued liquid at low temperatures while the pressure was main tained; but on removing the pressure (breaking the tube), they instantly passed into the gaseous state. In his memoir, published in the Philosophical- Transactions for 1823, he announced that he had succeeded in liquefying chlorine, euchlorine, sulphureted hydro gen, nitrous oxide,cyanogen, ammonia, and hydrochloric, sulphurous, and carbonic acids. Subsequently, by the joint action of powerful mechanical pressure and extreme cold, the number of liquefiable gases was so far extended as to include all except oxygen, hydrogen, nitrogen, nitric oxide, and coal-gas; and the following gases were obtained in a solid form—hydriodic acid, hydrobroime acid, sulphurous acid, sulpburetal hydro gen, carbonic acid, cyanogen, ammonia, euchlorine, fluoride of silicon.
The researches of Andrews established the fact that for every gas there is a certain minimum temperature at which the energy of the molecular movement is exactly bal anced by the force of cohesion, whatever be the pressure to which, the vapor is suldeeted; this temperature is the "critical point" of the gas. It was because the critical points of certain gases are very low that they so long resisted all efforts to condense them. No amount of pressure without the necessary cold could be effectual. At last, in the end of 1877, by the use of powerful apparatus and ingenious contrivances for produc ing cold, the difficulties have been overcome by MM. Cailletet and Raoul Pietet of Gen eva. By combining a cold of 120° to 140° below zero, with enormous pressures of 550 and even 650 atmospheres, M. Pietet was able to liquefy oxygen. `• He has also lique fied and even solidified hydrogen, which he has seen to issue from the tube in the form of a steel-blue liquid jet, which partly solidified. The solid hydrogen, in falling on the floor, produced the shrill noise of a metallic hail, thus confirming the bold and inge nious idea: ofFaraday, who first suggested that hydrogen is a metal." The distinction between permanent and condensable gases is thus abolished.