223 
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v/i : ' T:?'-p for a lang time. But in veflels of metal, this 
p • facia, m does not. t -ke place; fo that it is very plain 
ti- : the or! by f ' fiance, or filex, which is precipitated 
1. the comae’. o, the 'fluoric acid gas and the water, is 
only a portioned'the glafs which is attached and aCtually 
diiiolved by the aeriform acid. 
Info abcil-jar filkd with fluoric acid gas, and refting 
"n:ion mercury, lprinkle a little water over the lurface of 
this metallic fluid: heat is produced, and the mercury 
i in the jar. It often happens alfo, when this gas is 
pu: into water, that the; filex falls down in a quartzous 
pellicle ; each bubble of the acid, as it touches the wa¬ 
ter, is covered with the filex, leaving in its way, to rife 
to the lurface, a trace in the fhape of tubes, which 
Frieftley calls organ pipes, tapering upwards to a point, 
becaufe the bubble diminilhes in proportion as it is dif- 
folved by the water, and that the fllex is thus taken 
away. The filex, which firft falls down in the veflels, is 
foon after re-diflolved by the excels of the acid, in pro¬ 
portion as the water is Saturated; for the water, at firft 
imperfe&ly faturated with the acid, has not ftrength to 
hold the filex in diflolution. 
Bergman obtained fluat of filex cryftallized. When 
the fluoric acid is manufactured in veflels of glafs, filex 
is thereby thrown down, then dill'olved again by the re¬ 
action of the acid : this is true fluat of filex, inftead of 
pure fluoric acid. Alkalis may be ufed to afeertain the 
prefence of the filex. The tafte of this acid diflolved in 
water, is like fulphuric acid diluted with water, or vinegar. 
If a iolution of fluoric acid in water be expofed to 
the fire, part of the acid flies off in vapours 5 but the laft 
molecules adhere fo ftrongly, that the water and the reft 
of the acid will volatilile if the heat be increaled in pro¬ 
portion. The fluoric acid has no action on gold, filver, 
lead, mercury, tin, antimony, bifinuth, cobalt, nickel, 
arfenic, and manganele, in their reguline ftate. It dif- 
folves them, however, in that of oxyds. Iron and zink 
diffolve in this acid ; and their folution is accompanied 
by the evolution of hydrogen gas. This acid fiiould be 
preferved in bottles coated with wax melted in oil, or in 
veflels of lead or platina. 
The property of the fluoric acid gas for corroding 
glafs, gave M. de Puymaurin the idea of engraving on 
glafs, by ufing this acid in the fame manner as nitric 
acid is ufed for copper; but the acid mult be very pure, 
for, if at all containing filex, it will not eat into the glafs. 
For engraving on glals, cover the plate with wax melt¬ 
ed in oil; trace the figures to be engraved witha pointed 
inlirument through the wax; then expofe it to the va¬ 
pour of this acid gas. Guyton has availed himfelf of 
this property for engraving bottle-labels, efpecially for 
thof'e containing acids, which always burnt the paper 
ones. Thus has a new art been created, which may one 
day become highly valuable. The elements of this acid 
are as yet entirely unknown. 
Of BORACIC ACID. 
Boracic acid was firft made known to us by Homberg, 
a German phyfician : he called it fedative fait, or narco¬ 
tic fait, and employed it in medicine, as poffefling vir¬ 
tues both fedative and narcotic : but the London college 
has with much propriety rejeCled it. 
This acid is obtained from borax, or borat fuperfatu- 
rated with foda. To extraCt itfrom borax, take a faturated 
diflolution of this fait heated and drained; pour in ful¬ 
phuric acid by little and little, very gently and.carefully, 
for a brifk dil'engagement of caloric takes place, which, 
driving the water fuddenly.into vapour, makes a report, 
crackles, and might end in an explofion: take care alfo 
to put in enough of the acid, that the liquor may ac¬ 
quire a flight acidity. The fulphuric acid leizes upon 
the foda, and the boracic acid is precipitated by the cold 
in little fpangles of a very white colour. 
The fecond way is by lublimation. Put into a retort 
or cucurbit of glafs, with a head, feme borax, with fui- 
[ S T R Y. 
phuric acid and a little water; heat the mixture till it 
melts ; by means of the water, a part of the acid, fepa- 
rated from the borax by the fulphuric acid, rifes in va¬ 
pours, which are condenled and depofited as they grow 
cold, in the upper part of the apparatus, under the form 
of very bright thin filvery-white feales. To purify it, 
diflolve it warm, and it will cryftallize. Wafti it alfo 
in cold diftilled water, in order to feparate the acid fui- 
phat of foda. 
This acid is du&ile under the tooth, reddens the blue 
vegetable colours; is inodorous. It bladders up with 
heat; the water of cryftallization is difengaged ; in this 
ftate it is called calcined boracic acid.. In a greater heat 
it melts, and forms a white folid glafs, provided the 
experiment is made in a crucible of white clay; for, if 
the crucible be coloured, the glafs will be fo likewife. 
This glafs, upon expofure to the air, becomes opake, 
and covered with a light while powder. It is ufed in 
the compofition of pafte jewels. If this glafs be diffolved- 
in water, after being reduced to powder, it cryftallizes 
in fpangles. It is of great fixity, and is not volatilized 
by fire; but it changes the crucible, which Ihews that it 
is in fufion ; -it mull alfo be very dry. This fixity has 
made it fuppofed that it was an artificial produCi or 
compofition; for it has beCn remarked that Ample bo¬ 
dies are in general volatile. 
The boracic acid is not changed by the aClion of at- 
mofpherical air, whether it be hot or cold, moift or dry s 
the cryftals remain bright, ihining like pearls. It is fo- 
luble in water, requiring twelve parts of cold w r ater to 
one of the acid : three or four parts of hot or boiling wa¬ 
ter will be enough ; and this is the method of obtaining 
it cryftallized. It will volatilize by dillillation with wa¬ 
ter ; but itmuft be of a thick confluence, or in an aque¬ 
ous diflolution it will not volatilize at all. Sublimation 
cannot take place but in above 8o°, which can only be 
endured by a body almoft folid, not one which is liquid, 
aqueous, or little faturated. This proves that fixed bo¬ 
dies, when pure, become volatile when united withfub- 
ftances which enjoy this property to a certain degree, 
and that, in fuch cafe, the water feems to have a itrong 
attraction for the boracic acid. The fpecific principles 
of this acid are not' known. It is ufed in many chemical 
operations as a flux, under which character welhall here¬ 
after have occafion to mention it. 
Of EARTHY SUBSTANCES. 
Nature prefents to us feveral kinds of earths. No fuch 
thing as elementary earth is now admitted; for, inftead 
of one, feveral earthy fubftances have been afeertained, 
which have each an equal right to be called the elements 
fince each enters into the compofition of feveral bodies. 
Among the earths which are known, there are feveral 
which approach the alkaline form : Fourcroy calls theie 
falino-terreftrial fubftances, faline earths, alkaline earths, 
and terreftrial alkalis. 
There are now reckoned eight terreftrial fubftances, 
called earths: viz. filex, alumine, glucine, zircone, mag- 
nefia, lime, barytes, and ftrontian. 
Each of thefe earths is diftinguifhed by fpecific charac¬ 
ters, befides thole which belong to them in common, 
which Fourcroy terms generic. The generic characters 
are drynefs, not being changed by fire, infufibiiity, not 
to be decompofed, and in combinations to remain as Am¬ 
ple and indeltruClible fubftances. 
Humboldt difeovsred, that not only vegetable earth, 
but alfo clay, drawn from a great depth, and efpecially 
Ample earths, have the property, by mere contaCt with 
atmofpherical air, of depriving it of all .its oxygen. 
Alumine, barytes, and moiftened lime, will each let at 
liberty pure azot; and this is a new method of meafuring 
air more aCtive than pholphorus and fulphuret of potalh. 
Earth feems not to aCl upon air in the dry ftate; and 
hence moiftened magnelia and filex do not prelent the 
fame phenomena as alumyie. 
SlLEXo 
