EXPLANATION OP SCIENTIFIC TERMS. 



tessellated with thick tables irregularly 

 piled and grown together. It has a white 

 milky colour, but in its divided portions 

 it is usually transparent. This mineral 

 is found in the iron mines of Uto, in 

 Sudermania, a province of Sweden. 



AQUEOUS VAPOUR (Latin aqua, water) 

 is the vapour of water. See Vapour. 



ARC OF A CIRCLE. See Angle. 



ASYMPTOTES OF AN HYPERBOLA. 

 See Conic Sections. 



ATMOSPHERE (Greek atmos, vapour, 

 and sphaira, a globe) is that sphere of air 

 which surrounds and includes the earth, 

 and is the common receptacle of all its 

 vapours and exhalations. Its height, 

 taken as that to which the vapours are 

 supposed to ascend, or that where the rays 

 of li^ht cease to be reflected, is generally 

 understood to be between forty and fifty 

 miles. 



ATMOSPHERE, PRESSURE AND 

 WEIGHT OF. The atmosphere, like 

 other bodies, gravitates towards the earth. 

 It has consequently iceiyht and pressure. 

 The pressure upon every square inch of 

 the earth's surface is equal to the weight 

 of a column of the whole height of the 

 atmosphere, an inch square. This weight 

 varies with the elevation of the ground 

 and the fluctuating density of the air ; 

 but it is found, at an average, at the level 

 of the sea, to be about fifteen pounds ; 

 and, as fluids press equally in all direc- 

 tions, according to their heights, the 

 same pressure is exerted on the square 

 inch of the surface of every body, on 

 this globe, to which the atmosphere has 

 arcess. 



ATMOSPHERES, OXE,TWO,THREE, 

 &c. The elasticity of air increases with 

 its condensation ; and, tbe ordinary pres- 

 sure being fifteen pounds on the square 

 inch, a condensation which produces a 

 pressure of thirty pounds on the inch is 

 termed two atmospheres ; that which 

 gives forty-five pounds pressure is three 

 atmospheres, and so on. Pressures arising 

 from other causes, such as the weight of 

 liquids and the force of steam, are also 

 frequently counted by atmospheres. 



ATTRACTION (from the Latin attra- 

 here, to di aw to) is a name given to that 

 tendency which bodies have to approach 

 one another, when no obvious cause is 

 recognised. It differs from gravity in 

 being a more general term ; gravity is a 

 species of attraction. See Gravity. 



CAPILLARY. See Ca- 



pilhiry Attraction. 



CHEMICAL. See Che- 



rical Attraction. 



of COHESION. See Co- 



AXIS OF AN ELLIPSIS, PARABORA, 



&c. See Conic Sections. 

 AXIS OF REFRACTION. See Refrac- 



five Power. 



AXIS OF A CONE. See Cone. 



BALANCE is a lever, turning on a pivot 

 or fulcrum, constructed for the purpose 

 of finding the weight of different bodies. 

 The lever, or rod, of a balance is termed 

 the beam, and the parts of the beam on 

 each side of the pivot on which it turns 

 are its arm*. When those arms are equal, 

 it is the common balance ; and its ends, 

 to which the body to be weighed and its 

 equivalent counterpoise are hung, are 

 called the points of suspension. Other 

 kinds of balances, as the Roman balance, 

 or steelyard, the Danish balance, &c. 

 are described in Mechanics, Treatise ii. 

 chap. v. 



BAROMETER (Greek baros, weight), an 

 instrument for measuring the varying 

 weight of the atmosphere. It is parti- 

 cularly described at pp. 6 14 of the 

 Treatise on Pneumatics. The vacant 

 space at the top of the tube is called the 

 Torricellian vacuum, from Torricelli, the 

 inventor of the instrument. 



BERYL. See Emerald. 



BODY is any determinate part of matter. 



BOILING is that rolling, bubbling appear- 

 ance which water and some other liquids 

 assume, when they are converted, by 

 means of heat, into steam or vapour. It 

 is also termed Ebullition. 



BOILING-POINT. When a thermometer 

 is immersed in any particular fluid that 

 is in a state of ebullition, the point of the 

 scale of the thermometer which marks 

 the measure of heat, in that boiling fluid, 

 is its boiling-point. This point varies 

 with the nature of the fluid and the 

 pressure of the air under which it boils ; 

 but the boiling-point of a fluid, generally 

 speaking, is that degree at which ebulli- 

 tion is produced under the medium weight 

 of the atmosphere. 



BURNING-GLASS is a glass lens which 

 refracts the rays of the sun into a focus. 

 The solar rays may be also brought to a 

 focus by reflexion from a concave mirror, 

 then called a burning mirror. 



CAIRNGORM, a species of quartz. See 

 Quartz. 



CALCAREOUS SPAR is a crystallized 

 carbonate of lime. One of the purest 

 varieties has the name of Iceland spar, 

 though it is not peculiar to that island. 



CALORIC (Latin color, heat) is an ima- 

 ginary fluid substance supposed to be dif- 

 fused through all bodies ; and the sen- 

 sible effect of which is termed heat. With 

 chemists, caloric is, properly, the matter 

 producing the sensation, and heat the 

 sensation itself. The terms, however, 

 are often confounded, the word heat being 

 used both for the cause and the effect. 

 Caloric produces other effects besides the 

 sensation, namely, the expansion, rare- 

 faction^ and liquefaction of bodies. 



CONDUCTORS OF. See COM- 



ductors of Caloric* 



B 2 



