AIDE-DE-CAMP

An officer appointed to attend a general officer in the field, in winter-quarters, and in garrison, to receive and carry ordiers. A field-marshal is entitled to four, a lieutenant-general to two, and a major general to one. The king appoints as many as he pleases, and this situation gives the rank of colonel. AIGRETTE, in Botany. See Pappus. AIR, Atmospheric. (Gr. ai^j, air.) The air which surrounds our globe to a height of about forty miles, and which is e ential to all living beings, was one of the elements of the ancient philosophers: its weight and several of its mechanical properties were discovered by Galileo and Torr i cell i about the middle of the seventeenth century; but its composition was not accurately determined till more than a century afterwards. The air is transparent, colourle , inodorous, and tastele , e ential to the respiration of animals and vegetables, and to the support of combustion. It is 816 times lighter than its bulk of water; 1000 cubic inches, at mean temperature and pre ure, weighing about 305 grains. The air is a mixture of nitrogen and oxygen gases, with a small portion of carbonic acid gas and of the vapour of water: in particular situations, other substances exist in it; as, over marshes, miasmata; over sulphureous springs, sulphuretted hydrogen; over and near the sea, in dry weather, muriatic acid, either free or combined; and a substance, probably of organic origin, which, aided by light, reddens solution of silver; peculiar organic combinations, sometimes infectious, where people, especially the sick, are confined; sulphurous acid and ammonia, in London and other places where large quantities of coal are burned; and traces of nitric acid during severe thunder-storms. The leading constituents of the air are nitrogen and oxygen, which are to each other in the relative bulks of about 79 and 21, or 80 and 20; and these proportions are probably not liable to any appreciable change, either dependent on season, wind, weather, situation, or height from the surface. BerthoUet found 21 per cent, of oxygen in Cairo and in Paris; Sau ure, the same in Geneva; De Martyr, in Catalonia, and in all winds, weather, seasons, and states of the barometer; in wet and dry, and in [s. 38]