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1901), pages 711-24. For special memoirs, see Mendenhall, "Report on Atmospheric Electricity," Memoirs of the Xational Academy (Washington, 1889) ; Exner, Beitriige zur Erkentniss der at- mosphari.ichen Elektrizitiit, a series of memoirs in the successive volumes of the Sitzungsberichte of the Vienna Academy of Sciences ; J. J. Thom- son and C. T. R. Wifson, numerous articles in the Proceedings of the Philosophical Society of London: Elster and (Seitel, numerous memoirs in the Sitzungsberichte of the Vienna Academy of Sciences; Elster, a general report published in Terrestrial ^[agne^ism (Baltimore, 1900); and also in the Memoirs of the International Co7igress of Physics (Paris, 1900).

ATMOSPHERIC EN'GINE. See Caloric Engine.

ATMOSPHERIC INTFLUENCE. Air consists normally of oxygen, nitrogen, water-vapor, and carbonic "acid ; but owing to the continual decomposition of organic bodies that is taking place in nature, its constituents usually include many other gases, as hydrogen disulphide, sulphur tri- oxide, etc., which may act chemically on various bodies, and thus gradually cause their destruc- tion. The destructive effects of atmospheric elec- tricity and changes of temperature may also be included among the influences that tend to de- stroy various materials.

Atmospheric influence is conspicuously shown on buildings and other structures that are ex- posed to the air. The atmosphere of large towns usually contains an excess of carbon dioxide gas, and, where coal is burned, of sulphur. The re- fcistance of many building materials to atmos- pheric influence is, however, very great. Granite is regarded as the most stable of building-stones. Egj'ptian porphyry is also remarkably enduring. Basalt disintegrates unequally according to the amount of feldspar that it contains. The dur- ability of slate is in proportion to its density. Sands"tone, millstone grit, and conglomerates are affected through the decomposition of the mate- rial cementing' their particles, or the mechanical effect of moisture, as by freezing. Limestone de- cays with varying degrees of rapidity. The ap- plication of melted paraflin to structures that tend to disintegrate has been found of value in protecting them from decay. A notable example of atmospheric influence is presented by the E,g}-ptian obelisk in Central Park, New York, which had withstood the ravages of centuries in a dry atmosphere, but is rapidly disintegrating in the moist climate of Xew York. The power of brick, tile, and the like to resist the influence of the atmospliere is dependent on the chemical com- position of the material and the amount of burn- ing in their manufacture. If they contain lime they tend to crack and crumble under moisture. Much care is usually exercised in the proper se- lection of materials in making cements, and those best adapted to resist atmospheric influence are naturally selected. Wood and timbers are easily influenced and, when moist and exposed to cur- rents, resulting in rapid evaporation, cracks en- sue from shrinkage. Dry-rot is the result of ex- posure to high temperature with consequent im- prisonment of natural moisture ; while common •wet rot is the result of air and water combined. For the protection of wood, ordinary oil paint is the usual preservative. Treatment with special preparations, such as kyanizing, creosoting. and pickling in mineral salts, is also not infre- quently resorted to. (See Wood Preserving.) In the case of metals the influence is somewhat complicated by chemical and physical changes. Iron, when exposed to the influence of the atmos- phere, absorbs oxygen from the air and becomes rusty or coated with iron oxide. For its protec- tion, as in the case of bridges, a suitable paint is the most satisfactory preservative. Zinc, when exposed, similarly becomes coated with oxide of zinc, which serves to prevent further oxidation. Copper likewise is soon coated with an oxide that serves as a protecting agent. Glass, which is deficient in silica, deteriorates in consequence of the decomposition of its potash and soda. Paint- ings, statuary, and other works of art, as well as books and manuscripts, readily decay under the influence of the atmosphere. The bindings of books in libraries disintegrate in consequence of the sulphur that is often contained in illumi- nating gas.

ATMOSPHERIC RAIL'WAY. See Pneumatic Dispatch.

ATOLL, a-tol'. The name given by the Malays to a coral reef which forms an annular island, inclosing a lake of water. Some atolls are nearly 100 miles in circumference, and have from 15 to 60 fathoms of water. They make excellent har- bors, with safe entrances, always on the wind- ward side. Some of the reefs sustain consider- able vegetation, and are inhabited. See Coral Island and Coral Reef.

CEOS8-SECTION AND BIRD S-EYE VIEW OF AN ATOLL.

AT'OM. See Divisibility; Chemistrt; Molecules — Molecular Weights ; Atomic Weights : !M.^ttek.

ATOM, THE HISTORY AND ADVENTURES OF AN. A romance by Smollett (17(J9), satirizing the English political parties of the day.

ATOMIC THE'ORY. See Chemistry.

ATOMIC WEIGHTS. If one should weigh out a number of volatile oxygen compounds in such quantities that the volumes occupied by them ( under some given pressure and temperature) should be equal to one another as well as to the volume occupied by 2 parts by weiglit of hydrogen gas, he would find, by analysis, that some of these compounds contain 16 parts of oxygen, others 32 parts of this element, still others 48 parts of it, etc. The numlier of parts of oxygen in any one of the compounds would be either 16 or some multiple of this number; but in no case less. A similar comparison of the compounds of chlorine would show that they contain either .3.5.5 parts of this element, or some multiple of 35.5, but in no case less. By extending investigations of this kind to volatile compounds in general, chemists have been able to demonstrate that for every element there is a characteristic number representing the smallest amount of that element capable of existing in a certain fixed