' FLU 
especially when hot, and the ease with 
which it holds silica in solution, even when 
in a state of gas. This affinity for silica is 
so great that thp thickest glass vessels can 
withstand its action only a short time. The 
order of its affinities is, 
Alumina Potash 
Ammonia Silex 
Barytes Soda 
Lime Strontian. 
Magnesia 
As fluoric acid produces an insoluble com- 
pound with lime, it may be employed to de- 
tect the presence of that earth when held 
in solution. Two or three drops only of 
the acid will cause a milky cloud or preci- 
pitate to appear, if any lime is present. 
Fluoric acid has been applied to engrav- 
ing or etching on glass, and was used, ac- 
cording to Beckman, nearly a century and. 
a half ago for that purpose, by an artist at 
Nnremburg, who obtained it from digest- 
ing floor spar in nitric acid. Since, how- 
ever, the discoveries of Seheele and Priest- 
ley, it has been more generally used, and 
the art is performed by covering the glass 
with w'ax, and then that part where the fi- 
gures are to appear is laid bare, and the 
whole is exposed for some time to the hot 
vapour of fluoric acid. This simple process 
is employed with great advantage in writ- 
ing labels on glass vessels, and in gradu- 
ating thermometers, &c. See Thomson’s 
Chemistry. 
FLUSTRA, in natural history, horn- 
wrack, a genus of worms, of the order 
Zoophyta. Animal a polype, proceeding 
from porous cells ; stem fixed, foliaceous, 
membranaceous, consisting of numerous 
rows of cells united together, and woven 
like a mat. About eighteen species have 
been described. 
FLUTE, an instrument of music, the 
simplest of all those of the wind kind. It 
is played on by blowing it with the mouth, 
and the tones or notes are changed by stop- 
ping and opening the holes disposed for that 
purpose along its side. The ancient fistulas, 
or flutes, were made of reeds, afterwards of 
wood, and last of metal; but how they 
were J^lown, whether as our flutes, or as 
hautboys, does not appear. 
Flute, German, is an instrument in- 
tirely different from the common flute. It 
is not, like that, put into the mouth to be 
played, but the end is stppt with a tam- 
pion or plug ; and the lower lip is applied 
|o a hole about two inches and a half, or 
FLU 
three inches, distant from the end. This 
instrument is usually about a foot and a half 
long ; rather bigger at the upper end than 
the lower; and perforated with holes, be- 
sides that for the mouth, the lowest of 
which is stopped and opened by the little 
finger’s pressing on a brass, or sometimes 
a silver key, like those in hautboys, bas- 
soons, &c. Its sound is exceedingly sweet 
and agreeable ; and serves as a treble in a 
concert. 
FLUX, a general term made use of to 
denote any substance or mixture added to 
assist the fusion of minerals. In the large 
way, limestone or fluor spar are used as 
fluxes ; but ip small assays, the method of 
the great operations is not always followed, 
though it would be very frequently of ad- 
vantage to do so. The fluxes made use of 
in assays, or philosophical experiments, con- 
sist usually of alkalies, which render the 
earthy mixtures fusible, by converting them 
into glass ; or else glass itself into powder. 
Alkaline fluxes are either the crude flux, 
the white flux, or the black flux. Crude 
flux is a mixture of nitre and tartar, which 
is put into the crucible with the mineral in- 
tended to be fused. The detonation of the 
nitre with the inflammable matter of the 
tartar is of service in some operations ; 
though generally it is attended with incon- 
venience, on account of the swelling of the 
materials, which may throw them out of 
the vessel, if proper care be not taken either 
to throw in only a little of the mixture at a 
time, or to provide a large vessel. 
White flux is formed by projecting equal 
parts of a mixture of nitre and tartar, by 
moderate portions at a time, into an ignited 
crucible. In the detonation which ensues, 
the nitric acid is decomposed, and flies off 
with the tartarous acid, and the remainder 
consists of tire potash in a state of consi- 
derable purity. This has been called fixed 
nitre. 
Black flux differs from the preceding, in » 
the proportion of its ingredients. In this 
the weight of the tartar is double that of 
the nitre ; on which account the combustion 
is incomplete, and a considerable portion of 
the tartarous acid is decomposed by the 
mere heat, and leaves a quantity of coal 
behind, on which the black colour depends. 
It is used where metallic ores are intended 
to be reduced, and effects this purpose, by 
combining with the oxygen of the oxide. 
There is danger of loss in the treatment 
of sulphureous ores with alkaline fluxes : 
for, though much or the greater part of the 
