166 
CHEMISTRY. 
der, in a few hours, on the contact of air fevcral degrees 
below the freezing point. Some, as iron, copper, lead, 
burn flowly, and in the courfe of feme months, in the 
air, even though cold. All metals increafe their weight 
during this operation, which does not take place without 
the contaft of air, and confequently abl'orb a principle, 
the oxygen of the atmofphere, without lofing any one. 
Neither the name of calcination, which was given to this 
phenomenon, nor that of metallic calces, can be retain¬ 
ed; but inftead of thefe have been fubftituted the terms 
of combultion and oxydation for the operation, and of 
metallic oxyds to denote the metals thus burnt. The 
colours which metals difplay in burning, or with which 
their flame is tinged, appear to be owing to the diflolu- 
tion of the metallic molecules in the light that is evolv¬ 
ed. Thus copper yields a green flame, &c. 
Not only do all metals compared with each other ab- 
forb different quantities of oxygen to fatiirate them in 
their combultion by the contaft of air, but each metal 
confidered feparately ablbrbs different proportions, and 
flops at various points of oxydation, according to the 
degree of temperature to which it is railed. Thus tin, 
lead, copper, iron, change colour and aflume the tints 
of the rainbow, at the firfl: degree of Are to which they 
are expofed in contaft with the air: lead firfl: becomes 
a grey oxyd, next yellow, and laltly red : mercury poffes 
from black to white, from white to yellow, and from 
yellow to red : iron, at firfl: a black oxyd, becomes next 
green, then brown, and ultimately white: copper is at 
rirll: a brown oxyd, from which it changes to blue, and 
its iaft degree of oxydation imparts to it a green colour. 
All metals differ in their attradfion for oxygen. From 
fome, as gold, filver, &c. oxygen is elicited by means of 
light almolt alone, or affifted with a very final! portion 
of caloric ; others, as mercury, require for its reparation 
a great degree of lire, and much light; while the greater 
number do not part with this principle, merely by the 
intervention of light and caloric. To decompofe oxyds 
of the lafl: defeription, they muff be heated with char¬ 
coal, wliich attradts from them their oxygen. It is ow¬ 
ing to this diverfity of attraftive power for oxygen, that 
fome metals are deprived of it by others, as filver and 
gold are by almolt all the reft, mercury by copper, cop- 
} er by iron, &c. We are not yet perfedtly acquainted 
with all thele degree's of attradfion, but the prelent date 
of our knowledge indicates the following order of the 
metals, beginning with that of which the attradfion for 
oxygen is molt powerful: Manganele, zink, iron, tin, 
copper, mercury, lilver, gold. 
Various metals decompole water, and this the more 
fenflbly and rapidly the higher their temperature is raif- 
ed, becaufe then the abundant quantity of caloric em¬ 
ployed more forcibly attradts and dilfolves the hydrogen. 
Thus iron decompofes w r ater with great rapidity when 
it is of a white heat, though it requires a confiderable 
time toeffeft its decompofition at the higheft temperature 
our atmofphere ever attains. Iron, zink, tin, and anti¬ 
mony, appear to be capabie of decompofing water : it is 
probable, too, that manganefe, and even fome other me¬ 
tallic jubilances, are equally capable 1 of effefting this 
purpofe. This decompofition is attributable to a ftronger 
attradfion for oxygen than hydrogen poffeffes: whence 
it follows, that hydrogen completely decompofes the ox¬ 
yds of thofe which do not decompofe water. But it is 
requifite here to diftinguifh the different degrees of oxy¬ 
dation: for the oxyd of iron highly oxydated, or oxy- 
dated brown, is partly decompolcd by hydrogen, and 
reverts to the ftate of black oxyd, becaufe iron is capable 
of eliciting oxygen from water only to that degree which 
oxydates it black, and beyond this degree of oxydation 
decompofes it no longer. All metals capable of decom¬ 
pofing water effeft this operation with more eale and ra- 
pklity, when affifted by the contaft of a fubftance which 
has a great tendency to unite with their oxyds. Fre¬ 
quently even metals, like other combuffible bodies, 
•which alone would not decompofe water, are rendered 
capable of it by the prefence of fome other lubliance, 
which afts by a predilponent affinity. Thus almolt all 
metals are enabled to effeft the decompofition of water 
by means of acids. 
Metallic oxyds have this peculiarity in their combina¬ 
tions, that they feem, with regard to acids, to perform 
the functions of alkalis, or earthy and alkaline bales; 
though, on the other hand, they are capable of uniting 
with earths and alkalis, as if they were a fort of acids. 
Of the latter, indeed, there are fewer than the former; 
and it is obfervable, that they which faturate alkalis in 
the manner of acids, are in general thofe to which oxy¬ 
gen molt powerfully adheres, as antimony, lead, iron, 
and manganele. It has already been laid above, that 
that there are three metals which are truly acidifiable. 
Metals cannot be diffolved by acids without being pre- 
vioufly oxydated: for this reafon, fuch metallic oxyds 
as are foluble in acids, diffolve in them flowly, and with¬ 
out effervefcence; while metals themfelves cannot be 
diffolved in the lame menftruum, without motion and 
effervefcence being produced. The effervefcence occa- 
fioned by the diffolution of metals arifes from this, that, 
in abforbing oxygen, they take it from a principle which 
affumes the form of gas, or eiaftic fluid. This principle 
proceeds either from the water, or from the acid, em¬ 
ployed in the procefs, according as the one or the other 
is decompofed ; and fometimes it originates from both, 
when both are decompofed at the fame time by the me¬ 
tal. Sulphuric acid thus decompofed by metals, when 
in a concentrated ftate, gives out fulphureous gas; and 
nitric acid, nitrous gas. Sulphuric acid diluted with 
water, greatly facilitating the decompofition of the aque¬ 
ous fluid by means of metals, evolves in this procefs hy¬ 
drogen gas : this is eminently the cafe in the diilblution 
of iron or zink by dilute fulphuric acid. The phofpho- 
ric acid afts nearly in the fame manner as the lulphuric 
with metals. Nitric acid is not only decompofed by 
feveral metals, but alfo admits the decompofition of wa¬ 
ter at the fame time. For this it fuffices, that the metal 
diffolved in it be extremely greedy bf oxygen : fuch is 
particularly tin. In this cafe, the hydrogen of the water, 
uniting with the azot of the nitric acid, forms ammo¬ 
niac; for which reafon thefe foiutfons afford no gas, and 
contain ammoniacal nitrat. Hence we underlbmd how 
moft folutions of the white metals in nitric acid evolve 
ammoniacal vapours in the injeftion of quicklime. 
The muriatic acid, as it is incapable of being decom¬ 
pofed by combuffible bodies, of itfelf dilfolves few me¬ 
tallic fubftances. It attacks only fuch metals as poflefs 
fuflicicnt avidity for oxygen to decompofe water: ac¬ 
cordingly, during the diilblution of metals in the mu¬ 
riatic acid, hydrogen gas is always evolved. And not 
only is the muriatic acid incapable of being decompofed 
by metals, but it alfo pofleffes the property of attrafting 
oxj'gen from molt metallic oxyds: in doing which it 
panes-to the ftate cf oxygenated muriatic acid. To this 
jtrong affinity for oxygen the muriatic acid owes its pro¬ 
perty cf eafiiy diffolving metaliic oxyds, on which ac¬ 
count it is employed with fuccefs for diffolving the oxyd 
of iron, which other acids are unable to attack. If me¬ 
tallic oxyds be furcharged with oxygen when we dif¬ 
folve them in muriatic acid, an eflervelcence arifes, be¬ 
caufe a part of the acid flies off in the form of oxyge¬ 
nated muriatic acid gas. If the oxyds be only at the 
proper point of oxydation for uniting with this acid, 
they dillolve in it without any movement or effervel- 
cence, as fait or l'ugar will in water. The boracic and 
fluoric acids unite but feebiy with metallic oxyds : they 
do not diflbive pure metals, becaufe they are not de- 
compofable by them ; but they enable water to oxydate 
fuch of them as have moft affinity for oxygen. It is the 
fame with the carbonic acid, which unites w'ell with 
b , moft 
