SCIENTIFIC TERMS. 141 



Diamond, the most valuable, the most pellucid, and the hardest of all 

 the minerals. The Diamond is simply crystallized carbon. It is 

 very nearly the same as common charcoal, only it is in a state of 

 crystallization. 



Diaphanous Body, a body through which the rays of light can pass. 

 Dioptrics, the science of refracted vision. See page 27. 

 Discharger, an instrument used in electricity for the purpose of dis- 

 charging an excited body. 



Distillation, that process whereby the most subtle parts of a compound 

 body are separated by the means of caloric from the mass, in a 

 state of vapour. 



Divergent, a term in optics to express rays of lights receding from 

 each other ; concave glasses make the rays diverge, while convex 

 glasses make them converge. 



Ductility, a property possessed by various metals of undergoing ex- 

 tension. Gold is the most ductile, and lead is the least ductile of 

 such metels as possess this property. 

 Dynamics, the science of the motion of bodies. 



Earths, certain bodies which are incombustible and generally unalter- 

 able by fire. 



Echo, reflected sound. See page 51. 

 Effervescence, that motion which takes place in certain liquids through 



the escape of a gaseous substance. 

 Efflorescence, a kind of mealy substance on the surface of certain 



bodies, produced generally by decomposition. 



Elastic fluids, a name given to vapours and gases. Vapour is an elastic 

 fluid, but it may be condensed. Gas is a permanently elastic fluid, 

 because it cannot be readily condensed. 

 Electric, a body which may be easily excited by electricity, as resin. 



silk, wood, &c. 

 Electricity. See page 58. 



Electrometer, an instrument to measure electricity. See page 63. 

 Electrophorus, an electrical instrument for showing perpetual electri- 

 city. See page 62. 

 Emerald, a precious stone of great beauty and value. The colour of 



the emerald is generally green, sometimes it is sky blue. 

 Eudiometer, an instrument for measuring the purity of the air. 

 Evaporation, that act by which fluids are converted into vapour by 



heat. See page 53. 



Fata Morgana, or Mirage, an aerial phenomenon exhibiting in the at- 

 mosphere a part of a coast or country. \Vhen the weather is calm 

 there arises a vapour which acquires considerable density ; this is 

 sometimes so disposed as to receive, and reflect as from a mirror, 

 various objects. In this way the coast of France, some few years 

 since, became clearly visible from Hastings, although actually below 

 the horizon. 



Fermentation, an intestine motion arising spontaneously among the 

 particles of a mixed body, producing a different combination of those 

 parts : there are three kinds, acetous, vinous, and putrefactive. 

 Flame, volatile inflammable matter in the act of combustion. 

 Fluoric Acid, a peculiar acid obtained from fluor spar ; it is the only 

 acid that will corrode glass. 



