AGATE. A name given to many com binations of chalcedony, carnelian, quartz, amethyst and flint. It is one of the varied forms of silicions minerals, and contains 98 per cent. of silica ; it is opaque, and has a resinous fracture with deep tints, produced by traces of iron. When a section is made it displays a series of dark lines or bands, sometimes irregular, some times rounded, which are the edges of successive deposits made by the mineral during its formation. Agates take a high polish, and are much valued as ornamen tal stones in the manufacture of cups, rings, seals, knife-handles, snuff-boxes, &c. ; burnishers are made of agate for the use of bookbinders and silversmiths. These gems occur naturally in amygdaloid trap rocks, lying in nodules, surrounded by chloritic clay, also in beds of streams and rivers where they have been wash8d down. They are made darker in tint by being boiled in oil, and then dipped in oil of vitriol. The carnelian is an agate of a flesh red or yellow tint : it is com mon in the sandy plains f Africa, India, and Asia Minor.
AIR. The gaseous envelope of the earth. Our planet has two coverings: one the water which is distributed as lakes and seas, filling up the deep cavities of the solid surface, tending to produce a more level superficies ; the second covering is the air or atmosphere which restA upon the top of the water and the dry land, enveloping the highest mountains, and rising upwards to an altitude somewhat above 45 miles ; it is a true aeriform ocean surrounding our earth, and has upon its upper surface waves and tides, and throughout its mass, currents flowing in constant and variable direction's, precisely as those of the ocean comport themselves ; it is held down to the surface of the earth by attraction, and rotates with the planet ; its density varies with its actual height at the place of observation, of widen the barometric pressure is the evidence. This pressure dnninishes as the elevation above the sea increases, owing to the up per portions of the atmosphere pressing upon and condensing the lower strata so much so, that oneLhalf the actual weight of the atmosphere is comprised within the space of the lower 5 miles of its total height, the remaining 40 miles in height containing the other half. The air is highly compressible and elastic, and its volume diminished inversely as the pressure increases. This accounts for the facility of setting it in motion and its ve locity. Like fluids, it presses equally in every direction, and when it comes in contact with a more expanded, and there fore lighter portion of air, it pushes it up and occupies its place, producing currents of air and winds when it flows in streams, and sound when it is thrown into vibra tions or undulations. The air is warmed solely by the earth, and not by the trans mitted rays of the sun,—hence warm air exists within the tropics, and diminishes toward the poles, and sensibly decreases every 850 feet of elevation. Air was one of the simple substances of the ancient philosophers ; but it has been shown by Scheele and Cavendish to be a compound body made up of oxygen gas and nitro gen. The proportions in which they are found to exist, are 21 of oxygen and 79 of nitrogen by volume in 100 parts. These substances are not chemically united, they are merely mixed together. There is also contained in the atmosphere a small quantity of carbonic acid, amount ing to one twenty-five hundredth part, which, no matter at what elevation the air may be drawns still found. Saussure M detected it at Mont Blanc, and Boussin gault on the Andes, so that it is a regular constituent. Liebig has shown that am
monia can also be detected in the atmos phere, to which may be added a variable quantity of watery vapor, odors of plants, and other volatile substances ; it no doubt also contains floating particles (miasmata), during periods of epidemic disease. The chemical properties and the beneficial effects of the air are due to the pressure of oxygen, the removal of which, or any alteration of its amount and condition, renders the air injurious to life. (See VENTILATION.) Although air is and much lighter than solid or fluid bodies, yet it is still subject to all the physical laws which govern gases ; in a large quantity, as when the sky is clear, it gives us a blue tint to the eye, which may be due to the vapor of water in the atmosphere refracting the light. It occu pies a given space, and is impenetrable, and no other substance can occupy where it is except it be by displacement. It is ca pable of communicating weight : and 100 cubic inches of it are found to weigh 31.0117 grains at the temperature of 60°, and the barometric column standing at 30°. This weight is equal, on the whole atmosphere of 45 miles height, to a pres sure of 15 lbs. on every square inch. This pressure varies in different places and at different times. (See BAROMETER.) This pressure is exerted upon every sub stance at the level of the sea. Air may be compressed into a smaller volume, in proportion to the pressure exerted upon. Doubling the pressure condenses the air into one-half its bilk; when released from pressure it expands to its original bulk : this is due to its elasticity, which, like all eses, is very great.
Al U M P-,KXHAUSTING SYRINGE.
Instruments founded upon the elastic property of the air. The syringe consists of alarass cylinder with an air-tight piston ; a valve at the top opens upwards into the body, and one at the lower part opens outward (at the side) into the external air. This apparatus is screwed on to any vessel which requires to have the air re moved. On raising the piston the air from the vessel below follows it upward, filling the cylinder ; if the lower stopcock be now closed, and the cylinder pressed down, the air will escape by the valve at the side, and the cylinder can be emptied in this way. By constant repetitions of raising the piston, and then expelling the contained air of the cylinder, the greater part of the air of the attached vessel is drawn up and removed. The whole air cannot be discharged in this way : for after it has been worked some time, and the greater quantity of air discharged, the elastic force of the remainder is so slight as not to be able to raise the valve. The air-pump is a doubly exhausting syringe, which has its valves in the pis ton or plug. There are two moving in the cylinder or barrels, with a reciprocat ing motion communicated by a toothed wheel and racked piston rods. The bar rels communicate by means of a tube with a table of metal, upon which is fixed a bell-glass or receiver, made stoutly, and with a strong rim at the bottom ground finely, so as to fit smooth on the table ; a a little tallow or fat is used to smear the table to make the fitting more tight. The receiver is thus a transparent air-tight chamber in which any object may be placed from which it is needful to remove air. A stopcock is fitted to the connect ing tube, to stmt off or let on external air when desired. To good air-pumps a mercurial gauge is attached.