ATT 



AUG 



times as great as that of an atom of hy- 

 drogen, and half as great as that of an 

 atom of sulphur. On this theory are 

 founded three universal laws : — 



1. That the parts hy weight in which 

 bodies mutually combine, stand in a fixed 

 numerical relation to each other. 



2. That in every chemical combination 

 the parts hy weight of one element may be 

 represented by certain weights of other 

 substances. 



3. That the iveights which mutually 

 represent one another at once furnish the 

 proportion in which these equivalent sub- 

 stances combine. 



ATRA'CHIA (a, priv., rpdut, to per- 

 forate). A tribe of bivalved MoUusca, 

 which are destitute of siphons for im- 

 bibing food. See Macrotrachia. 



ATRACTE'NCHYMA ( arpa/CTor, a 

 spindle, e-(xviJia, an infusion). A term 

 applied by some botanical writers to the 

 fusiform, or spindle-shaped, variety of 

 spheroidal cellular tissue. 



A'TROPOUS (a, priv., TpeTrco, to turn). 

 That which is not inverted, as applied in 

 Botany to the erect ovule. In this case, 

 the parts of the ovule undergo no altera- 

 tion of position during their growth, and 

 thus the two sacs and the nucleus are 

 all connected at the base of the ovule. 

 The term is synonymous with orthotro- 

 pous. 



ATTE'NUATE {ad, to, tenuis, thin). 

 Tapering ; gradually diminishing in 

 oreadth, and terminating in a point. 



ATTENUA'TION (attenuo, to make 

 thin). The process by which a fluid 

 becomes of less specific gravity, as when 

 it undergoes fermentation, and parts 

 with carbonic acid. 



ATTRACTION {attraho, to draw to). 

 A general term denoting the mutual 

 tendency of bodies towards one another, 

 and explanatory of certain physical and 

 chemical properties of matter. 



1. Attraction of Cohesion. The ten- 

 dency of the particles of bodies to cohere, 

 and form masses. On the degree of this 

 force depend the three aggregate forms of 

 matter, which are distinguished as the 

 solid, the liquid, the gaseous. Cohesion 

 is the antagonist of affinity. 



2. Attraction of Gravitation. The mu- 

 tual tendency of the masses of bodies 

 towards one another. Universal gravi- 

 tation relates to the mutual action of the 

 celestial bodies on each other and the 

 gravitation of our earth ; terrestrial 

 gravity relates to the mutual operation of 

 terrestrial objects, or to the effects which 



43 



the earth produces on all bodies more 

 immediately connected with it. Cohesion 

 operates at sensible, gravitation at insen- 

 sible distances. 



3. Attraction of Affinity. The tendency 

 of the atoms of dissimilar bodies to com- 

 bine and form chemical compounds. 

 This force effects the combination and 

 decomposition of dissimilar bodies, with 

 the production of individual properties 

 distinct from those which they had in 

 their former state. See Affinity. 



4. Attraction, Capillary. The power 

 by which a liquid ascends in the interior 

 of a capillary tube, or tube of small bore, 

 above the surface of the liquid which 

 surrounds it. The phenomenon occurs 

 in solid bodies which are capable of being 

 wetted. 



5. Attraction, Superficial. A term 

 sometimes applied to that modification 

 of the adhesion of bodies which is caused 

 not merely by the mass of bodies, but 

 also by the number of points of contact, 

 or the extent of their touching surfaces. 



6. Attraction, Electrical. The force 

 which is exhibited in the tendency of the 

 free positive and negative electricities to 

 regain their equilibrium by combination 

 each with its antagonist. Hence, elec- 

 trified bodies attract all other non-elec- 

 trified bodies, and all oppositely electrified 

 bodies, in order to saturate themselves 

 with the opposite electricity and regain 

 electrical quiescence. 



7. Attraction, Electrodynamical. A 

 term expressive of Ampere's discovery, 

 that *' the electricities in a state of mo- 

 tion, i.e. as electric currents, act attrac- 

 tively and repulsively on each other 

 according to a certain law," in a manner 

 resembling the polar attraction of statical 

 electricity, i. e. of electricity in a state of 

 tension. 



8. Attraction, Magnetic. The property 

 by which bodies endowed with magnetism 

 attract certain metals; display towards 

 one another a force partly attractive, 

 partly repulsive ; and exhibit a tendency 

 to arrange their mass in a certain direc- 

 tion. 



ATTWOOD'S MACHINE. A ma- 

 chine invented by Mr. Attwood for cal- 

 culating exactly the velocity of a falling 

 body from an actual measurement of its 

 height, and the time which it takes to 

 reach the ground. 



AU'GITE {av-yh, lustre). Pyroxene. 

 A simple mineral, of a dark-green or 

 black colour, with an internal shining 

 lustre, forming a constituent part of 



