AIR, ATMOSPHERIC. From the remotest antiquity, until a comparatively recent period, the air was considered as one of four elements, of which all things were compounded ; and it was generally held to be an invisible, impon derable, and simple substance. Some of the ancients, it is true, had vague notions of its gravitating power and of its elasticity ; amongst others may be mentioned Aristotle, who says, that a bladder filled with air weighs more than when quite empty; and Hero also says, that the air, in a given cavity, may be rarefied by sucking out a part of it ; but still their ideas were very imperfect, and their opinions were abandoned by their followers, for various absurd hypo theses by which they attempted to account for the phenomena produced by the action of the atmosphere. It was not until near the middle of the seventeenth century, that the real nature and properties of the atmosphere were ascertained with and confirmed by experiment. For these discoveries we are principally indebted to Galileo, and his pupil Torricelli. Galileo taught that the air had weight, but does not appear to have applied this idea to explain those pneumatic phenomena which were then absurdly attributed to an imagi nary principle of nature, termed a horror of a void : but Torricelli following up the principle, chewed, by incontrovertible experiment, that the rise of fluids in pumps was owing to the pressure of the atmosphere, and that the height to which fluids would rise in vacuo, was exactly proportionate to their weight, which would, in all cases, be exactly equal to a column of air of the same base, and of the height of the atmosphere. These great principles were successfully followed up by various eminent philosophers, and numerous important con clusions and useful applications of them were the result. Towards the close of the nineteenth century, the air was discovered to be a compound of two gases, to which the names of oxygen and azote were given, and this discovery served as the basis of the modern system of chemistry, which has been so fertile in brilliant scientific results. Under the articles CHEMISTRY and PNEUMATICS
will be found the demonstrations of the chemical and physical properties of the atmosphere ; we shall, therefore, in this place, merely state what are its prin cipal characteristics, and the uses to which they are made subservient in many processes of the arta and manufactures. Air, then, is an invisible elastic and ponderable gaseous fluid, its bulk and density depending upon its temperature, and the pressure to which it is exposed. Under a pressure of 30 inches of mercury, and at a temperature of 60° Fahrenheit, its spec. gray. as ascertained with great care and accuracy by Messrs. Arago and Biot, is .00122, water being 1.00000, or it is 820 times lighter than water; and if we take a cubic inch of water to weigh 252.525 grains, then 100 cubic inches of air will weigh 30.808 pains, and a cubic foot will weigh 532.36 grains. Mariotte ascertained that its bulk is inversely as the pressure, a double pressure reducing a given volume to half its bulk. The applicability of this law to air under very great pressure, has been questioned, but not satisfactorily disproved ; and for all practical pur may be safely received; whilst, frcm its simplicity, it may be remem with ease and applied with facility. Air expands by an increase of temperature, the rate of expansion is not exactly equal for equal increments of heat, but on an average, the increase of bulk above 32° Fahrenheit is of its bulk, for each degree of heat on the same scale. The expansion or rare faction of air is accompanied by a decrease of its temperature, and in its conden sation or compression, it gives out a proportional quantity of heat. Air can take up and hold in solution a portion of water depending upon the temperature.