C H £ M I 
moft metallic oxyds, and is often found combined with 
them in their native ltate. 
Metallic acids are eaiily decompofed by very combuf- 
tible metals: they unite very perfeftly with their oxyds, 
and are frequently found thus combined in a native ltate. 
Acids of the vegetable and animal kind, or of which the 
radicals are formed by hydrogen and carbon, are not de¬ 
compofed by metals; but they render water extremely 
decompofable by them, and unite with metallic oxyds 
with conliderable folidity. Many of them occafion thefe 
oxyds to revert to the metallic ftate. 
The oxyds of metals cannot unite with acids, and ftill 
lefs remain combined with them, unlefs they contain 
certain proportions of oxygen: if they have lei'sthan the" 
determinate quantity, no union will take place; if more, 
they will feparate from them. Belides this general truth, 
there is another of the lame kind peculiar to each acid 
and each metal; which is, that they cannot remain re¬ 
ciprocally united, but within certain limits of oxydatron, 
and thefe oftentimes very confined. There is a deter¬ 
minate proportion of oxygen in the combination of an 
acid with a metallic oxyd. It is in confequence of this 
law, that metallic lolutions expofed to the air grow tur¬ 
bid, and forma precipitate, in proportion as the metallic 
oxyd, by abforbing oxygen from the atmofphere, be¬ 
comes gradually inloluble in the acid. This is the rea- 
fon of the decompofition, which the atmofphere effects 
on moft metallic lulphats and nitrats. It even frequent¬ 
ly happens, that metallic oxyds diflblved in acids readt 
by degrees on thefe liilts, and take from them, though in 
clofe vefleis and without the contaft of air, a portion of 
their oxygen, I'o that they foon feparate, and are precipi¬ 
tated to the bottom of the Iblution. Heat is eminently 
favourable to this fucceflive decompofition of acids by me¬ 
tallic oxyds. Thus nitric lolutions, when heated, grow 
turbid, or become more and more decompofable by airand 
water, which is particularly remarkable in the nitric Iblu¬ 
tion of mercury. There are metals which have lo great 
tendency to oxydate tbemfelves with acids, that they can • 
not remain united, or form permanent lolutions with 
them. This is particularly the cafe with thole, which 
have the property of becoming acids, or forming oxyds 
capable of .combining with alkalis: as arsenic, tunglVen, 
molybdena, antimony, tin, iron, &c. accordingly we find 
the lolutions of thefe metals, elpecialiy in the nitric acid, 
are always loaded with precipitates, and contain little or 
no metallic oxyd. 
From what ha-s been faid it appears, that, to form me¬ 
tallic falts, the oxyds ot metals muft remain united with 
acids, and have no tendency to feparate from them. It 
is requisite too, that we do not augment their affinity for 
oxygen, or bring them into contaft with this principle. 
Metallic compound lalts have always, or almoft always, 
an excefs of acid: all of them likewile are more or l<|fs 
acrid or corrofive, which Ihovvs a tendency to becoffie 
acid in moft metallic oxyds. Thole properties of metal¬ 
lic falts with which it is cfimportance to be acquainted, 
may be included under the following heads, i. Figure, 
and its varieties. Sapidity or caufticity, more or lefs 
powerful. 3. Alteration by means of light. 4. Fiifion, 
defecation, decompofition, by means of caloric, more or 
lefs marked. 5. Deliquefcence, tffiorefcence, or decom¬ 
pofition, more or lefs complete, by the action of the air. 
6. Solubility in water, warm or cold; decompofition 
more or lefs promoted by pure water, Sec. 7. Decompo- 
fition by earths and alkalis; mture of the metallic oxyds 
precipitated; complete precipitation, or formation of tri¬ 
ple falts, partly alkaline or earthy, partly metallic. 8. 
Alteration of the metallic oxyds precipitated, at the in- 
' ftant of their precipitation, either by the air, or by the 
nature of the alkali employed for the precipitation, as 
happens when ammoniac is ul'ed. 9. Reciprocal altera¬ 
tion by different acids; decompofition taking place, or 
not; affinity of acids for the metallic oxyds ; changes of 
the oxyds difcoverable by their colour, jo. Alteration 
S T R Y. 167 
by earthy or alkaline neutral falts, whether exhibiting 
an union without decompofition, or a double decompo- 
iition. 1 r. Reciprocal aftion of metallic falts on each 
other, announcing either limple union, a finiple change 
of bales by the acids, or a dilplacement of oxygen preci¬ 
pitating both the oxyds; one hecaufe it is partly dil- 
oxydated, the other btcaule it is fuperoxydated, as .is the 
cale, for example, in the ufeful precipitation of the mu¬ 
riatic Iblution of gold by the muriatic folution of tin, 
which furniflies the purple precipitate of Caflius. 12. 
Union with earthy or alkaline fulphures ; the formation 
of akind of a fulphurous ores. 
Metallic oxyds have different degrees of affinity with 
acids, and fome may be employed to decornpole combi¬ 
nations of others. But the different affinities of metals 
for oxygen are the moft important caufe of the pheno¬ 
menon of the precipitation of metallic lolutions. Thus 
feveral metals, by taking oxygen from others diflblved in 
acids, occalion their re-appearance in the metallic form ; 
as mercury does with filver, copper with mercury, iron 
with.copper, zink with iron, See. Sometimes metals do 
not deprive metallic oxyds dilfolved in acids of all their 
oxygen; which occurs when the precipitating metal has 
no occalion for all the oxygen of the metal dilfolved, to 
aflume its place in the acid : thus tin, when it precipitates 
the oxyd of gold, does not elicit from it all the oxygen it 
contains, but fuifefs it to precipitate in a peculiar ftate of 
oxydation. Metallic oxyds, in dividing oxygen.among 
them in new proportions, precipitate with properties 
which deferve to be more accurately inveftigated- than 
has hitherto been done. From the fabts here laid down, 
we are enabled to inveftigate the preparation. of ail the 
metallic oxyds ufeful in the arts: coloured glafs, enamels : 
metallic lalts, of ufe in the arts : the elfebts of thefe falts 
in the arts in which they are employed : the folution and 
parting of metals : the precipitation of metallic oxyds by 
alkalis and earth. Thefe applications are in general lo- 
uleful and multifarious, that they cannot be exhibited 
unlefs in the particular detail of each metal. 
THE FORMATION AND NATURE OF VEGETABLE SUB¬ 
STANCES. 
The fubfhmces which conftitute the texture of vegeta¬ 
bles differ from mineral lubftances in this, that they, are 
of a more complex order of compofirion, and, though 
all are extremely fufceptible of decompofition or analylis, 
not one is an object of fynthefis. Nothing but the tex¬ 
ture of living vegetables, nothing but their vegetating 
organs, can form the matters extracted from them;. and. 
no inftrument invented by art can-imitate the comgofi- 
tions, which are formed in the organic machines cf plants. 
Though vegetables form all the materials-, which con¬ 
ftitute their texture with four or five natural lubftances, 
caloric, light, water, air,.and the carbon derived from 
fome remains of plants-decayed into mould, we find an 
extreme variety in the properties of thefe materials'. 
Thefe may be reduced, however, to a certain number of 
principal heads, under the name-of immediate materials 
of plants, becaufe they are obtained from them by limple 
proceffes, almoft wholly mechanical, by. a fpi t of cliredt 
analylis, which does not alter their nature. Thefe mat¬ 
ters, which are more or lefs compound, are placed in 
particular organs, veffeis, diftinif cells, See. Sometimes 
their feat is in the root, or ftalk, bark, and leaves, at the 
fame time : at others they are contained only in the flow¬ 
ers, fruits, or feeds, and even in certain -parts of thele 
organs. This particular fitualion of the immediate ma¬ 
terials indicates the different organization of the texture 
of the part, as the caufe of the various nature dilplayed 
by each of them. The different place occupied by each 
of thefe materials of vegetables, often enables us to ob¬ 
tain them eaiily feparate and pure. It is lufficient, when 
this local diltribution occurs, to bruife and open the 
veffeis or cell’s which contain them, and exprefs their li¬ 
quid juices. Nature Lerfelf frequently exhibits this le- 
paratioa 
