ATM 



ATO 



compact and in an arenaceous form ; in 

 the latter, it is termed copper sand. 



A'THANOR. An ancient kind of fur- 

 nace, so constructed that the fire should 

 be constantly supplied wiih fuel in pro- 

 portion to the consumption. 



ATHERFCERA (u(^>;p, an ear of com, 

 KP()a<:, a horn). A section of Dipterous 

 insects, in which the antennae have only 

 two or three joints, the last being fur- 

 nished with a bristle. It includes the 

 Flies, strictly so called, the Bot-flies, &c. 



ATHE'RMANOUS (a, priv., 6>epMH, 

 heat). A term applied to those sub- 

 stances which retain all the heat they 

 receive, as distinguished from diather- 

 manous bodies, which transmit all the 

 rays of heat through their substance. 



ATMO'METER (ctTyuof, vapour, fx^rpov, 

 measure). An instrument for measuring 

 the quantity of exhalation from a moist 

 surface in a given time. 



A'TMOSPHERE (uTiuoc, vapour, 

 a-cpalpa, a sphere). That volume of air 

 which surrounds the earth. Its mean 

 height has been estimated at from 44 to 

 47 miles, its volume as the twenty-ninth 

 of that of the globe, and its weight only 

 forty-three thousandths. 



1. Atmospheric Pressure \s indicated by 

 the length of a column of mercury. A 

 mercurial column, 30 inches in length, 

 presses on a given surface with the same 

 force as the atmosphere in its ordinary 

 state ; and hence the force of a 60-inch 

 column is equal to the pressure of two 

 atmospheres ; that of 15 inches, to half an 

 atmosphere ; that of one inch, to l-30th of 

 the atmospheric pressure. 



2. Atmospheres— two, three, 8^c. Mul- 

 tiplied pressures of air arising from con- 

 densation, the ordinary pressure being 

 fifteen pounds on the square inch. Press- 

 ures arising from other causes, as the 

 weight of liquids and the force of steam, 

 are also frequently counted by atmo- 

 spheres. 



3. Atmosphere, refraction of. A term 

 expressive of the refraction of the rays 

 of light, as they pass from the celestial 

 bodies into the atmosphere, owing to its 

 being denser than the ethereal or vacuous 

 medium which intervenes between it and 

 the celestial bodies. This refraction 

 varies with the density of the atmo- 

 sphere, and also with the direction in 

 which the rays enter. The atmosphere 

 gradually increases in density from the 

 higher to the lower strata; and, there- 

 fore, a ray of light will be more and more 

 refracted in its passage to the earth's sur- 



42 



face, so as to descend in a curved line. 

 The curve, too, varies with the direction 

 of the ray, that coming from the zenith 

 alone being a straight line. 



4. Atmosphere, electrical. The dis- 

 tance at which an electrified body is 

 capable of resolving the neutral electri- 

 city of unelectrified bodies into its ele- 

 ments, when brought into proximity to 

 the former, is termed the electrical at- 

 mosphere of that body. 



5. Atmosphere, magnetic. The range 

 within which a magnet exerts its influ- 

 ence in any magnetizable bodj'- within a 

 certain distance of itself, is termed its 

 circle of magnetic influence, or its mag- 

 netic atmosphere. 



ATMOSPHERIC ENGINE. A term 

 applied to the single-action steam-engine, 

 in which only the ascent of the piston is 

 effected by means of steam, its descent 

 being mainly eflfected by the pressure of 

 the atmosphere. 



A'TOLLS. Coral-islands, or lagoon- 

 islands, of an annular Ibrm, or consisting 

 of a circular strip or ring of coral sur- 

 rounding a central lagoon. 



A'TOM (a, priv., reyuvco, to cut). An 

 ultimate particle of matter, incapable of 

 further division. Berzelius distinguished 

 atoms into elementary and compound. 

 The latter are subdivided into, 1. Com- 

 pound atoms of the first order, or atoms 

 formed of only two elementary sub- 

 stances united ; 2. Organic atoms, or 

 those composed of more than two ele- 

 mentary substances, and found only in 

 organic bodies, or bodies obtained by the 

 destruction of organic matter ; and, 

 3. Compound atoms of the second order, 

 or those formed by the union of two or 

 more compound atoms, as the salts. 



ATO'MIC THEORY. A theory for 

 explaining the nature of matter, founded 

 by Leucippus. He considered the basis 

 of all bodies to consist of extremely fine 

 particles, differing in form and nature, 

 which he supposed to be dispersed 

 throughout space, and to which his 

 follower Epicurus first gave the name of 

 atoms. To these atoms he attributed a 

 rectilinear motion, in consequence of 

 which such as are homogeneous united, 

 whilst the lighter were dispersed through- 

 out space. See Dynamic Theory. 



ATO'MIC WEIGHTS. An expression 

 connected with the theory that all bodies 

 consist of atoms, which are of the same 

 size and shape in the same body, but 

 which differ in weight in diflferent bodies. 

 The weight of an atom of oxygen is eight 



