EXPLANATION OF SCIENTIFIC TERMS. 



particular position with relation to ano- 

 ther magnet. 

 DIRECTRIX OF A PARABOLA. 



See Omic Sections. 



DISTILLATION is a process by which a 

 fluid, or portion of a liquid, is converted 

 into vapour by means of heat; and that 

 vapour returned into a liquid state by 

 cold, or, as some say, by the abstraction 

 of caloric. Distillation is evaporation, 

 that is, raising 1 a liquid to the state of 

 vapour ; but the latter term does not 

 include the idea of preserving that vapour 

 and condensing it again into a liquid. 

 The chief object of distillation is either 

 to separate mixed liquids, where the 

 steam of one rises at a lower heat than 

 the other; or to free a simple liquid from 

 dregs or impurities. In the distillation 

 of vinous liquors, alcohol is produced, 

 which boils at a much lower heat than 

 water; but it is not yet perfectly ascer- 

 tained whether this alcohol existed ready 

 formed in the liquor, or owes its forma- 

 tion to a chemical action during the pro- 

 cess. ' The grosser liquor remaining in 

 the still, after the spirit is all exhausted, 

 is called spent wash, and is food for hogs. 

 DIVERGING RAYS (of light) are the 

 opposite of converging (which see). They 

 separate in their progress further and 

 further asunder, as the radii of a circle do 

 from its centre. 



DODECAHEDRON. See Rhombus. 

 DOGMATISM See Empirical. 

 DOUBLE REFRACTION. See Refrac- 

 tion. 



DUPLICATE RATIO. See Ratio, Com- 

 pound. 



DYNAMICS (Greek, dynamis, force) is 

 that division of the science of mechanics 

 which considers bodies as acted upon by 

 forces that are not in equilibria. It, 

 therefore, treats of bodies in motion. See 

 Equilibrium. 



DYNAMETER (a Greek compound signi- 

 fying a measurer of power) is an instru- 

 ment so contrived as to measure, with 

 accuracy, the magnifying powers of mi- 

 croscopes and telescopes. It is described 

 at page 1 1 of the treatise on Optical In- 

 struments. 



EBULLITION. See Boiling. 

 ELASTICITY (from a Greek word signi- 

 fying to push, or drive back) is that qua- 

 lity of a substance, whether solid or fluid, 

 by which, when compressed, or when 

 forcibly expanded, it endeavours, in either 

 case, to reassume its former bulk. 

 ELASTIC FLUIDS. See Fluids and Gas. 

 ELECTIVE ATTRACTION. See Che- 

 mical Attraction. 



ELECTRICITY. If a dry piece of amber 

 be rubbed by the hand, it will acquire the 

 property of alternately attracting and re- 

 pelling light bodies, such as bits of straw, 

 paper, or leathers. This power continues 

 only for a short time, when it becomes 



exhausted and ceases to art. If the expe- 

 riment be made in the dark, sparks of 

 light will be observed, passing between 

 the light -bodies and the amber. Glass 

 and several other substances have similar 

 properties ; and the collection of pheno- 

 mena which have been observed concerning 

 them is called electricity, from the Greek 

 electron, amber. Bodies that are capable 

 of being endowed with this power of 

 attraction and repulsion and the emission 

 of light are termed electrics. They are said 

 to be excited by the friction ; and a ma- 

 chine contrived to produce this excitation 

 in an easy and rapid manner, by means 

 of a peculiarly-constructed cushion, called 

 a rubber, is an electrical machine. Bodies 

 that are not excitable by such means are 

 non-electrics. 



ELECTRIC FLUID. In order to ac- 

 count for the various phenomena of elec- 

 tricity, it has been supposed that there is 

 a subtile fluid, identical with lightning, 

 which pervades the pores of all bodies, 

 and is capable of motion from one body 

 to another. When the natural quantity 

 belonging to a particular body (then said 

 to be saturated} is increased, that body is 

 said to be positively electrified ; and, when 

 that quantity is diminished, it is nega- 

 tively electrified. These two states have 

 also been called plus and minus electricity ; 

 and the former being obtained from glass 

 and the latter from sealing-wax, the two 

 are sometimes distinguished by the terms 

 vitreous and resinous electricity. 



ELECTRICITY, ANIMAL. See Ga/- 

 vanism. 



, VOLTAIC. See Gal- 



} POSITIVE AND NE- 

 GATIVE. See Electric Fluid. 



- , RESINOUS AND VI- 



TREOUS. See Electric Fluid. 



ELECTRICS AND NON-ELECTRICS. 

 See Electricity. 



ELECTRICAL BATTERY. See Leyden 

 Phial. 



ELECTRO -MAGNETISM is a science 

 which comprehends the phenomena shew- 

 ing the connexion between Electricity 

 and Magnetism. 



ELECTROMETER, an instrument nsed 

 for ascertaining the quantity and quality 

 of the electricity in an electrified body. 

 See Index. 



ELECTROSCOPE, an instrument for ex- 

 hibiting the attractive and repulsive agen- 

 cies of electricity. See Index. 



ELIXIR VIT.E. See Alchemy. 



ELLIPSIS. See Cone and Conic Sections. 



ELLIPSOID. See Conoid and Spheroid. 



EMERALD. The emerald is ranked 

 among the 'gems, and is now found only 

 in Peru. It is of a green colour, rather 

 harder than quart/, and always in crys- 

 tals, which are translucent and generally 

 transparent. What is called oriental 



