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158 
than feven hundred parts of water; attracts carbonic 
acid from the atmofphere, and forms on the furface of 
its folution a pellicle of chalk, improperly called cream 
of lime-, is unalterable alone, but melts with filex and 
alumin; and is formed of principles with which we are 
yet unacquainted, though it is evidently a compound. 
The pretended converfion of earths, each into other, 
admitted by natural philofophers, is nothing more than 
a chimera: I'o far is it from being proved, that fdex be¬ 
comes alumin in confequence of expofure to the air, that 
flints are changed into chalk, and that chalk is convert¬ 
ed into magnelia, as fome have fuppofed on much too 
feeble grounds. T.he three alkaline earths are to appear¬ 
ance more manifellly compound than the other two. 
There is reafon to prefume, that azotis one of their prin¬ 
ciples, and gives them their alkaline properties : however, 
experience has yet afforded no proof in fupport of this 
opinion ; though their origin, al'cribed with confiderable 
propability to marine animals, which contain a large por¬ 
tion of azot in their compolition, renders it not dellitute 
of foundation. As to the metallic nature of the five 
earths, which fome fuppofed they had demonftrated by 
an imaginary reduftion of them into metals, by expofmg 
them to a violent heat in contact with Charcoal; the very 
finall and few metallic globules obtained, evidently came 
from the charcoal and the earth of the cupels, and has 
been difcovered to be nothing more than phofphure of 
iron, in the treatment of each of the different earth's; 
whence it is fufflciently proved, that they afford no me¬ 
tallic lubllance. Several natural pljiloibphers continue 
of opinion, that earths are fpecies'of burnt bodies, to 
which the oxygen is llrongly adherent, and incapable of 
being decom poled on account of their powerful attrac¬ 
tion for this, principle; but in this they are not fupport- 
ed by experience. 
Earths cpmbine.by twos, threes, and even greater num¬ 
bers, by proceffes unknown to us, which nature employs 
on a very extenfive fcale, to produce Hones differing in 
hardnefs, texture, tranfparency, opacity, colour, form,&c. 
If art has failed to imitate thefe compounds, the reafon is, 
time, fpace, and quantity, are wanting. Something fimilar 
to the natural earthy compounds, however, may be pro¬ 
duced, by leaving a long time in contact, earths, intimate¬ 
ly mingled, and moillened at firft with a little water. The 
three alkaline earths form a kind of intermediate link be¬ 
tween earths and alkalis. The latter are diftinguifhable 
by their acrid, burning, and urinous, tafte, their cauffici- 
ty, their Angular aftion on the fkin, and all animal fub- 
ftances, the quality of changing the blue colour of vio¬ 
lets to a green, and even a greenifh yellow', and deliquef- 
cency. We are acquainted with three fpecies, potafh, fc- 
da, and ammoniac. The firff and fecond have been called 
fixed alkalis, becaufe tlrey melt and grow red in the fire 
before they become volatile, the third has been named 
volatile alkali, from poffefling the oppofite property. 
Potafn is known by the following characters ;—It is 
dry, folid, wdiite, chryftalized in rhomboidal plates, fu- 
fible at a temperature of 203", and very deliquefcent, 
abforbs water with heat and a peculiar faint fmell, com¬ 
bines with Alex by fuAon, and forms with it atranfparent 
compound. It is frequently found native with lime, 
and combined with different acids; but is chiefly obtain¬ 
ed from vegetables, in theaflies of which it remains after 
combuffion. It is fuppofed that it bears fome analogy 
to lime, and may perhaps be formed of azot combined 
with it: experience, however, gives no fupport to this 
opinion. 
Soda is procured from marine plants by incineration, 
and confti.tutes the bafis of fea-falt. It ftrikingly relem- 
bles potafh in form, caurticity, fuiibility, deliquefcency, 
combination with Alex by means of fufion, aftion on 
animal fubffances, See. fo that it Was long confounded 
with it, and might have continued to be fo, if it did not 
form very different falts with acids, and yield thele acids 
to potafli. It has been imagined, that lodawas a com- 
a 
I S T R Y. 
pound of magnefia and azot, becaufe falts with foda for 
their bale have been as frequently found with falts hav¬ 
ing for their bafe magnefia, as calcareous falts with thofe 
having potafh for their bulls : but both thefe opinions 
remain hitherto equally devoid of proof. 
Ammoniac, or volatile alkali, differs greatly from the 
tw'o preceding fpecies in its form of gas when diffolved 
in caloric, in its liquid form when diffolved in wafer, in 
its pungent and fuffocatiiig fmell, its folubility in air, and 
its known and eafy decompofition by the eleftric (park, 
metallic oxyds, and the nitric and oxygenated muriatic 
acid. This decompofition evinces, that ammoniac is a 
compound of hydrogen and azot ; and for this reafon it 
frequently exhibits the phenomena of a combuffible lub- 
ftance. Hence alfo it is eafy to conceive, how animal 
matters furnifh ammoniac in confequence of putrefaction. 
If azot fliould at fome future period appear to be the 
principle that forms alkalis, the atmofphere will be found 
to confilt of a mixture of oxygen and alkaligen, each fe- 
parately diffolved in caloric; and it will exhibit an im- 
menfe fefervoir, from which the philolopher will perceive 
nature drawing the materials of two dalles of com¬ 
pounds, the molt aCtive and ufeful in a great number of 
her operations. From a conflderation of the faCts laid 
down under this head, we are directed to a comprehenfive 
view of the extraction, preparation, and purification, of 
earths: the theory of the arts of the potter-, brickmaker. 
See. the theory of cements and mortars: the reciprocal 
combinations of earths by means of fire: the natural hif- 
tory of Hones: the compound nature of earths and Itones: 
the alteration of colours by means of alkalis : vitrifica¬ 
tion, and the art . of making glafs : the extraction and 
purification of potafli and foda: the theory of alkaline 
cauftics: fome points regarding putrefadion, See. See 
the article Mineralogy. 
THE NATURE OF COMBUSTIBLE BODIES. 
Combuflible bodies are too various, too numerous, and 
important in the phenomena they exhibit, and the com¬ 
binations they are inceffaritly entering into with each 
other, and with the air, not to excite us to examine them 
with care, and endeavour accurately to alcertain their 
properties' and fpecific characters. In comprehending 
under this name all fubffances capable of combining 
more or lefs rapidly with oxygen, and difengaging from 
it caloric and light, we fhould arrange them in two clafles; 
fimple or indecompofed combuftibles, or fuch as cannot 
be decompofed, and combuftibles more or lefs compound. 
We call thole combuftibles fimple, which we are hitherto 
unable either to decompofe, 01 co compote by the union 
of different fubffances. With,their intimate nature we are 
unacquainted. Sometimes they occur flngly in the mi¬ 
neral kingdom, or in one of the others ; but they are 
found molt ufually combined two and two together. Such 
are the diamond, hydrogen, fuiphur, phofphorus, carbon, 
and the metals. Each of thefe lix genera muff be cou- 
fidered feparately. 
The diamond, is the hardeff of all the bodies that we 
know. It is very remarkable for the power with which 
it refraCts and decompofes light, from which Newton dif¬ 
covered that it was very combuflible. It is found native, 
cryffallized in oCtaedra, dodecaedra, &c. exhibits fome 
varieties differing in texture, denlity, and colour ; burns 
with a perceptible flame, and is converted into vaperur 
by burning. Its combination with oxygen is unknown. 
Few fubffances aft upon it; and, if it were not cornbuf- 
tible, it might be confidered as infufceptible of alteration. 
We know no compound into which it enters as a confti- 
tuent part; and it feems of all bodies the lealt obedient 
to chemical attraction. 
Hydrogen, as before noted, is one of the principles 
of water. With caloric and light it forms hydrogen gas, 
fixteen times as light as air, in folublein molt fubffances, 
capable on the other hand of diffolving fuiphur, phof¬ 
phorus, carbon, arfenic, oils, &c. and thus forming the 
different 
