Gas (Sax. gast, Ger. Geist, Dutch geest, spirit), a generic term used to designate any aeriform fluid which is neither liquefied nor solidified at ordinary temperatures and pressure, introduced by Van Helmont in the early part of the 17th century. Excepting the atmosphere, little was known of aeriform bodies by the ancients; but, under the name spiritas or flatus, artificial gas had been referred to by writers between the 14th and 17th centuries. It seems, however, to have been generally believed that such gases were only impure atmospheric air. Paracelsus noticed the evolution of gas by the action of oil of vitriol on iron as an eruption of air, but seems to have attached no special importance to it. Van Helmont was the first to attempt any systematic examination of gases, and to make a distinction between them and vapors; but his knowledge was necessarily imperfect, as he was not aware of the fact, long afterward demonstrated by Faraday, that most gases are condensible as well as vapors. The different gases are described under their respective titles. Most of their physical properties, and their absorption by liquids and solids, are treated of in the articles Heat, Pneumatics, Atmospiieee, and Absorption, together with notices of the principal discoveries pertaining thereto.

The laws of their chemical combination are treated in the article Atomic Theory. In the present article will be considered the general management and the diffusion of gases, and illuminating gas. I. Management of Gases. The collection and preservation of gases for experiment and observation may be effected by one of three methods, depending upon the nature of the gas and the mode of its generation. 1. It may be received in an exhausted vessel by means of a tube and stopcock. This method usually requires that the vessel be several times tilled with the gas and exhausted, to remove the residual air which always remains at the first exhaustion in consequence of inability to produce a perfect vacuum. Bags, which may be very nearly emptied of their contents, are often conveniently employed in this method of collection. 2. By displacement.

This is done by filling a bell glass with water in a pneumatic cistern, placing it on the shelf, and bringing the mouth of the tube delivering the gas beneath it, as represented in fig. 1.When the gas to be collected is easily absorbed by water, some other liquid is chosen, usually mercury. A modification of this plan, often used in collecting gases slightly absorbable by water, as hydrogen and oxygen, for ordinary experimental purposes, is to use a gas-holder, consisting of a copper cylindrical vessel, A, fig, 2, open at the top, in which is received a cylinder, B, closed at the top and open below, and counterbalanced by a weight attached to a cord passing over pulleys. By filling the outer cylinder with water, opening the stopcock c in the upper one, and depressing it, all the air may be forced out. Then, by at-taching the delivering pipe to the stopcock d in the outer cylinder, the gas will ascend into the inner one, which will rise as the pressure is restored to its interior.

When a strong jet is required for use, weights may be laid upon the inner cylinder and the counterbalance weights removed. Another form of gas-holder is represented in fig. 3. A drum of copper, A, has mounted upon it a shallow vessel, B, communicating by two tubes with stopcocks, g and A, one of the tubes passing to near the bottom of the cylinder, while the other only enters the top. A water gauge, e f,, shows the height of water in the drum; an opening at t admits the end of the tube supplying the gas, and a stopcock at c is for its exit. To use the apparatus, open the stopcocks, close the opening t with a plug, and pour water into the vessel B until the drum is filled; then close the stopcocks and remove the plug from the opening t. Atmospheric pressure prevents the water from fliowing out. Introduce the end of the tube supplying gas; it will ascend in the drum, displacing an equal volume of water, which flows out at t. When sufficient gas has been introduced, close the opening t, and open the stopcock g. The gas in the drum will then receive a hydro-static pressure equal to the height of the column of water in the tube and upper vessel above the level of water in the drum. The stopcock c may then be connected with any apparatus to which it may be desired to deliver the gas.

The forms of apparatus of this kind may be varied indefinitely, but these examples will suffice for illustration. When it is desirable to separate mixed gases, which are absorbable in different degrees by different liquids, or when it is desired to saturate a liquid with a gas, an apparatus called Woulfe's bottles (fig. 4) is often used. The gas is made to enter each bottle at a and to pass out at e. A safety and supply tube, s, passes through a middle neck to below the surface of the liquid. A cup at the upper end is for the purpose of receiving a portion of liquid which may be forced up the tube by any sudden expansion. The number of bottles employed may be varied according to the requirements of the case.

II. Diffusion of Gases. All gases, when mingled together mechanically in any proportion, tend to diffuse themselves uniformly, regardless of their specific gravities. Thus, if two bottles are connected together by an upright glass tube 10 or 12 inches long and about 1/20 of an inch in calibre, and the upper bottle is filled with the lightest of all gases, hydrogen, and the lower one with oxygen, whose specific gravity is 16 times that of hydrogen, or with carbonic acid, which is 22 times as dense, after the lapse of two or three days the two gases will be found to have the same proportion to each other in both bottles. This was the original experiment of Dalton, published in vol. xxiv. of the '"Philosophical Magazine." The same result was obtained by Berthollet with a tube 10 inches long and one fifth of an inch in calibre, when the apparatus was placed in a position which secured a uniform temperature, so that no motion could be communicated to the gases.