464 PLY POWDER 



the phenomena of this class, refers this effect to an alteration of the refraction of the 

 ray by the first surface upon which it falls. Sir John Ilorschel first drew attention 

 to this peculiar condition as exhibited in a solution of sulphate of quinine in water 

 slightly acidulated with sulphuric acid. Here we have a perfectly colourless solution 

 when we look through it, which sends back to the eye fine blue rays when we look 

 at the surface on which the solar rays fall. Sir John Herschel referred this to 

 epipolic dispersion, or dispersion from the first surface of the fluid on which the light 

 fell. There are many substances which appear to possess this property of altering 

 the refraction of rays, or are fluorescent. Beyond this brief explanation, we cannot 

 afford space in this dictionary to deal with the subject. We must refer those 

 interested to the Philosophical Transactions, in which Prof. Stokes's communications 

 appeared. 



. FXiUORXNX:. (Symbol, F. ; Atomic weight, 19.) The elementary base of hydro- 

 fluoric acid, which has never yet been isolated. 



The power of liberating a principle from fluor-spar which would etch glass was 

 known as far back as 1670; Scheole, in 1771, examined hydrofluoric acid, and 

 regarded it as an oxygen compound with an unknown element. Amp6re, in 1810, 

 determined that hydrofluoric acid was a compound of hydrogen and fluorine. 



Fluorine combines with most of the metals, and with hydrogen, boron, silicon, 

 sulphur, and phosphorus ; with chlorine, bromine, iodine, and oxygen, it exhibits no 

 tendency to unite. It has been suggested to employ hydrofluoric acid in the purifi- 

 cation of beet-root sugar ; the alkalis present in the juice being effectually removed 

 by the acid, while the organic matter remains unaffected. 



FIUOR-SPAR. (Chaux flwtee, Fr. ; Fluss-Spath, Ger.) This mineral often 

 exhibits a variety of vivid colours. It crystallises in the cubic (monometric) system, 

 with regular octahedral cleavages ; spec. grav. 3*14 to 3*19 ; hardness = 4'0 ; scratches 

 calc spar, but is scratched by a steel point ; usually phosphorescent with heat ; at the 

 blowpipe decrepitates and fuses into an opaque bead ; acted on by the acids with dis- 

 engagement of a vapour which corrodes glass ; its solution affords precipitates with 

 the oxalates, but not with ammonia. Its constituents are, fluorine, 487 ; calcium, 

 61-13 in 100. 



Fluor-spar occurs subordinate to metallic veins ; as to those of lead, in Derbyshire 

 and Cumberland ; of lead and copper, in Cornwall, and in Saxony and Bohemia ; 

 but it is found also in masses or veins, either in crystalline rocks, associated with 

 quartz, barytrs, &c., as in Auvergne, Forez, Vosges, Norberg in Sweden ; Norway ; 

 Petersburg ; Gouroek, in Scotland, &c. ; or among limestones, slates, and sandstones, 

 in Derbyshire, Cumberland, Cornwall, and New Jersey. It exists also in the 

 anvvgdaloids of Scotland, and in the volcanic products of Monte Somma at Vesuvius. 

 The variously -coloured specimens, called Derbyshire spar, are worked upon the 

 turning lathe into vases and other ornamental objects. 



A very beautiful variety, which has been much used for ornamental purposes, 

 known from its colour as 'Blue John,' has been obtained from Tray Cliff, near Castle- 

 ton, Derbyshire. The beautiful colour of the natural fluor has been successfully 

 imitated by exposing some of the common varieties to heat. 



Fluor-spar is employed to a considerable extent in the production of hydrofluoric 

 acid and for etching on glass. It is also used by lead smelters as a flux. The beautiful 

 phenomenon of fluorescence is so named from the fact that many of the fluor-spars 

 have the power in a high degree of thus affecting the rays of light. See FLUO- 



RESCEXCE. 



FIiUVIATIIiE (fluvius, a river), belonging to a river. 



FZiVX (Eng. and Fr. ; Fluss, Ger.) signifies any substance capable of promoting 

 the fusion of earths or metallic ores by heat. White flux is the residuum of the defla- 

 gration, in a red-hot crucible, of a mixture of two parts of nitre and one of cream of 

 tartar. It is in fact merely a carbonate of potash. Black flux is obtained when equal 

 parts of nitre and tartar are deflagrated. It owes its colour to the carbonaceous matter 

 of the tartaric acid, which remains unconsumod ; the quantity of nitre being too small 

 for that purpose. The presence of the charcoal renders this preparation a convenient 

 flux for reducing calcined or oxidised ores to the metallic state. Limestone, fluor-spar, 

 borax r and several earthy or metallic oxides, are employed as fluxes in metallurgy. 

 See METALLURGY. 



PX.Y POWDER. Under this name there is sold on the Continent the Mark 

 coloured powder obtained by the spontaneous oxidation of metallic arsenic in the air. 

 Various preparations of white arsenic are used for the same purpose in this country. 

 King's yellow is much used ; it thould bo made by boiling together sulphur, lime, 

 and white arsenic, but much that is sold is merely arsenic and sulphur mixed. 



Objecting on principle to the familiar use of arsenic and dangerous substances, 

 a preference may be given to a substitute for tho above, niado by boiling quassia chips 



