EXPLANATION OF SCIENTIFIC TERMS. 



9 



parative quantity of matter, in different 

 bodies, which is contained in the same 

 space. (See Folume.) Gravity is under- 

 stood to act in proportion to the relative 

 quantity of the matter of bodies ; and, 

 hence, the specific gravities of bodies are 

 presumed to be the measure of their 

 densities. See Gravity. 

 DE-OXVDATION is the depriving a sub- 

 stance of the oxygen, or vital air which it 

 contains. Concerning the de-oxydating 

 power of the solar rays, see Optics, p. 29. 

 DIAMETERS, TRANSVERSE AND 



CONJUGATE. See Conic Sections. 

 DIGESTER, a strong vessel of iron, or 

 other metal, having a screwed-down and 

 air-tight lid, into which substances, either 

 fluid or solid, are inclosed, and are therein 

 submitted to a much higher degree of 

 heat than they could be subjected to in 

 the open air. 



DIOPTRICS is that division of the science 

 of Optics which treats of the Refraction 

 of Liqht. 

 DIRECT PROPORTION, or DIRECT 



RATIO. See Ratio. 

 DIRECTION, LINE OF. See Force, 



Direction of, 

 DIRECTRIX OF A PARABOLA. See 



Conic Sections. 



DISTILLATION is a process by which a 

 fluid, or portion of a fluid, is converted 

 into vapour by means of heat, and that 

 vapour returned into a state of fluidity 

 by cold, or, as the chemists say, by the 

 abstraction of caloric. Distillation is 

 Evaporation, that is, raising a fluid to the 

 state of vapour, but the latter term does 

 not include the idea of preserving that 

 vapour and condensing it again into a 

 fluid See Vapour 



DIVERGING RAYS 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. 

 DOUBLE REFRACTION. See Refrac- 

 tion. 



DYNAMICS (Greek dynamis, force) is 

 that division of the science of mechanics 

 which considers bodies as acted upon by 

 forces which are not in eqnilibrio. It 

 therefore treats of bodies in motion. 

 See Equilibrium. 

 EBULLITION. See Boiling. 

 ELASTICITY (from a Greek word signi- 

 fying to push, or drive back) is that 

 quality of a substance, whether solid or 

 fluid, by which, when compressed, or 

 when forcibly expanded, it endeavours, in 

 either case, to re -assume its former bulk. 

 ELASTIC FLUIDS. See Fluids, and 



Go*. 



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 quartz, and always in crys- 

 tals, which are translucent and generally 

 transparent. What is called Oriental 

 emerald is a green sapphire. The Beryl 

 is a variety of the emerald, of a paler 

 green, frequently passing into blue, and 

 is much less prized. It is found in va- 

 rious countries, sometimes in Scotland. 

 The Emerald of Brazil is a Tourmaline, 

 which see. 



EMPIRICAL (Greek en andpeirao, I try) 

 designates any assertion or act which is 

 made or done, merely as an experiment, 

 without any past experience or known 

 principle to direct the choice. 

 EQUATION OF A CURVE. See Conic 



Sections. 



EQUILIBRIUM. When two or more 

 forces, acting upon a body, are so opposed 

 to each other that the body remains at 

 rest, although either would have moved 

 it if acting alone, those forces are said to 

 be in equilibria, which is a Latin term 

 signifying equally balanced. 

 ETHER. See Air, ethereal. 

 EVAPORATION; the state og Action of 

 a fluid when its particles are so tar sepa- 

 rated by caloric as to assume the form of 

 vapour." Evaporation, or (as it is some- 

 times called) vaporization, is often, but 

 not always, preceded by ebullition. See 

 Boiling and Vapour. 



E VOLUTE OF A CURVE. See Curves. 

 EXHAUSTED RECEIVER. See Va- 

 cuum. 



EXPANSIBILITY is that property of a 

 substance which renders it capable, under 

 certain circumstances, of occupying more 

 space than it usually requires. The grand 

 agent in the expansion of bodies is ca- 

 loric. 



FAHRENHEIT'S THERMOMETER is 

 that arrangement of the scale of the in- 

 strument, in which the space between the 

 freezing and the boiling points of water, 

 under a medium pressure of the atmo- 

 sphere, is divided into 180 parts, or de- 

 grees : the freezing being marked 32 

 and the boiling 212. This scale was 

 adopted by Fahrenheit, because he sup- 

 posed, erroneously, that 32 of those divi- 

 sions below the freezing-point of water 

 (which was therefore (0) on his scale) 

 was the zero, or greatest degree of cold. 

 See Thermometer. 



FELDSPAR is, next to quartz, the most 

 abundant stone that exists ; being a con- 

 stituent in granite and other rocks. It 

 scratches glass, and gives out sparks with 

 steel ; but all its varieties are inferior to 

 quartz in hardness. The transparent la- 

 minae of its crystals have a double re- 

 fraction ; and one of the species, Adularia, 

 or Moonstone, exhibits a pearly lustre. 

 It is the famous Petuntse of the Chinese, 

 being the vitrifying ingredient in their 

 porcelain. 





