G 



EXPLANATION OF SCIENTIFIC TERMS. 



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 various coun- 

 tries, sometimes in Scotland. The eme- 

 rnld of Brazil is a tourmaline, which see. 



EMPIRICAL (Greek en, and peirao, I try) 

 designates any assertion or act which is 

 made or done as an experiment, indepen- 

 dently of hypothesis or theory. The term 

 is generally used in a bad sense; especially 

 in the science of medicine, in which an 

 empiric is synoiiimous witli a quack. The 

 empiric is supposed to set up his own 

 short-lived experience to the collected 

 knowledge of ages, which is understood to 

 guide the regular practitioner ; and 

 which the empiric, in his turn, vilifies by 

 the name of dogmatism, a Greek deriva- 

 tive, denoting a fixed and positive opinion. 



EQUATION OF A CURVE. See Conic 

 Sections. 



EQUATOR, MAGNETIC. See Dipping 

 Needle. 



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 move it if 

 acting alone, those forces are said to be 

 in equilibria, which is a Latin term signi- 

 fying equally balanced. 



ERIOMETER, an instrument, invented by 

 Dr. Young, for the purpose of measuring 

 the diameters of minute fibres. It is de- 

 scribed in the treatise on Optics, pp.37, 38. 



ETHER. See Air, Ethereal. 



(in chemistry) is the name of an 



extremely volatile liquid, formed by the 

 distillation of some one of the acids with 

 alcohol. There are, of course, several 

 modes of production, according to the 

 acid employed ; as nitric ether, sulphuric 

 ether, &c. ; but, when well rectified, the 

 ether is the same whatever acid has been 

 employed. 



EUCLASE, a scarce species of emerald, re- 

 markably brittle ; and hence its Greek 

 name. 



EVAPORATION is the conversion of 

 water or other liquids into vapour or 

 steam, which becomes dissipated in the 

 atmosphere in the manner of an elastic 

 fluid. When the evaporation is sponta- 

 neous, that is, at an ordinary tempera- 

 ture of the atmosphere, and without arti- 

 ficial heat, it is commonly called exhala- 

 tion, whereas evaporation is often under- 

 stood to be produced artificially, and pre- 

 ceded by boiling. See Boiling, Steam^ 

 and Vapour. 



EVOLUTE OF A CURVE. See Curves. 



EXCENTRICITY. The distance between 

 the /bet of an ellipsis is called its excen- 

 tricity, and sometimes its ellipticity. It 

 is in this way that we speak of the excen- 

 tridly of the orbits of the planets which 

 are supposed to move in ellipses. 



EXCITATION. See Electricity. 



EXHAUSTED RECEIVER. See Va- 

 cuum. 



EXPANSIBILITY. See Gloss. I. 



FA H RE N H KIT' S THE IIMOMETER. 

 See Gloss I. 



FELDSPAR. See Gloss. I. 



FIRST, or PRIME MOVER, in mecha- 

 nics See Machine. 



FIXED ALKALIES. See Alkali. 



FLUID, ELECTRIC. See Electric Fluid. 



, MAGNETIC. See Magnetic 



Fluid. 



FLUIDITY. See Gloss. I. 



FLUIDS, ELASTIC, NON-ELASTIC, 

 &c. See Gloss. I. 



FLY-WHEEL. See Gloss. I. 



FOCUS. See Gloss. I. 



REFRACTED AND GEOME- 

 TRICAL. The point in which the rays 

 of light (according to their known laws) 

 ought to be concentrated, when reflected 

 from a concave mirror, or refracted 

 through a lens, is termed the Geometrical 

 Focus,- that in which they are actually 

 found is the Refracted Focus. These 

 foci are separated from one another in 

 proportion to the degree of spherical 

 aberration . 



FORCE. See Gloss. I. 



CENTRIFUGAL. See Centri- 



fugal, 

 petal. 



CENTRIPETAL. See Centri- 



DIRECTIONOF. See Gloss. I. 



COMPOSITION. See Gloss. I. 



ACCELERATION OF. See 



Acceleration. 



DIRECTIVE. See Magnets, Ar- 

 tificial. 



FORMULA, a short general form, or rule, 

 easily to be remembered, directing how 

 certain things may be done. The word 

 is a Latin diminutive, and has often the 

 plural formula in place of formulas. 



FREEZING-POINT. See Gloss. I. 



FRICTION. See Gloss. I. 



FRIGORIFIC. See Gloss. I. 



FULCRUM. See Lever and Balance. 



FUSION. See Gloss. I. 



GALVANISM, so named after its disco- 

 verer, Professor Galvani of Bologna, 

 " includes all those electrical '.pheno- 

 mena, arising from the chemical agency 

 of certain metals with different fluids*." 

 Volta discovered the means of multiplying 

 those effects : hence the science has also 

 been called J'oltaism ; and (from its action 

 on the muscles of animals newly killed) 

 Animal Electricity. 



GALVANIC CIRCLE. If, between two 

 plates of different metals, a fluid be inter- 

 posed which has a chemical effect on 

 one of the metals, and little or none on 

 the other; and if a communication be 

 made between other parts of the plates, 

 by means of a conducting substance (as a 



* For a definition which involves no hypothesis, see 

 Galvanism, 1, 



