CHE M I 
large mattes of air in the atmofphere, fire taken in gene¬ 
ral, water; and the earth, attended to at large, a. Or as 
combined ; and in this Hate they corifider the air, water, 
and earth, obtained from different bodies in their ulti¬ 
mate analyfis. 
Such were nearly the opinions adopted refpefting the 
principles of bodies, from the time of Beecher and Stahl, 
till the valuable difeoveries of Prieftley and Lavoifier, on 
fixed air and combination, neceflarily introduced new 
opinions. In fa£t, if immutability of properties, unity, 
and fimplicity, be the true characters of elements j and 
if it be admitted that this fimplicity no longer exifts, 
when a body is found to be capable of decompofition, it 
muft be remarked: i. That among the four elements 
there are at prefent two, namely, air and water, the prin¬ 
ciples of which, art has fucceeded in decompofing and 
feparating. a. That elementary earth is a creature of 
the imagination ; fince it is now abundantly fhewn, that 
there are many earthy fubftances equally fimple and in¬ 
capable of decompofition. From this general enumera¬ 
tion of fads hereafter to be fully explained, it follows, 
that the true principles, or firft elements of natural fub¬ 
ftances, efcape our fenfes and our inftruments ; that many 
of thofe which have been called elements, on account of 
their volume, their influence in the phenomena of na¬ 
ture, and their multiplied exiftence in its different pro¬ 
ducts, are very far from being fimple and unchangeable 
bodies 5 and that probably there is no body which falls 
under our fenfes which is fimple, but only appears 
fuch to us, becaufe we do not poffefs the means cf de- 
compofing it. Thefe affertions, moreover, agree with 
the opinions of fome of the ancient philofophers, who 
did not regard the elements as the moll fimple bodies, 
but fuppoted them to be formed of principles of afar 
greater degree of tenuity and unchangeablenefs. Thefe 
notions concerning thofe bodies, which have for fo many 
ages enjoyed the exclufive title of elements, and to which 
we deny that prerogative, do not prevent us from con- 
fid ering fire, air, earth, and water, as containing the 
principles of which moft natural bodies are formed, and 
of which we fhall now treat. 
OF FIRE. 
Among the four bodies anciently called elements, no 
one appears to be more aftive, nor at the fame time more 
fimple, than fire. The moft early philofophers, and af¬ 
ter them philofophers in every age, have given this name 
to a fubltance which they l'uppofed to be a fluid ex¬ 
tremely moveable and penetrating, formed of particles 
continually agitated, by them regarded as the principle 
of fluidity and of motion. When we refleCt on this fub- 
jeCt, we fhall find that thefe properties could oniy be at¬ 
tributed by conjecture to a body placed among the ele¬ 
ments, fince its exiftence has never been demonftrated j 
as that of the three other elementary fubftances has al¬ 
ways. been. It is, indeed, natural to think that this 
name, fire, has, in all languages and times, been given 
to the imprejfwn that heated bodies -communicate, or 
make on the fenfes : and which is fynonymous to the 
term heat, as well as to the light that bodies emit when 
in combuftion. This is the idea which the greater part 
of mankind entertains; they recognize the prefence of 
fire only by that of heat and combuftion. The chan¬ 
cellor Bacon is one of the firft who doubted the exiftence 
of fire as a peculiar fluid, and took notice that philofo¬ 
phers ; in defining it, had always miftaken a property for 
a feparate fubftance. Boerhaave, whole Treatile on Fire 
will always be regarded as a mafter-piece, was fenfible of 
this difficulty, and, in order to render the properties of 
this pretended element more evident, he examined its 
effeCls on bodies wherein it is thought to exift; fo that 
he, like all the philofophers who preceded him, has writ¬ 
ten a hiftory of heated, luminous, ratified, burning, bo¬ 
dies, rather than that of fire itlelf. This confulion is 
likely to be always found in natural philofophy j for the 
S T R Y. 179 
properties of fire are neceflarily connected with thofe of 
the bodies whereon it acts; fo far from having it in our . 
power to infulate it, we cannot even form an idea of its 
feparate exiftence : and, notwithftandirig the advanced 
ftate of chemiftry, it has not been found poflible to feize 
and confine this principle, which philofophers feem agreed 
to call a fluid, and whofe effects they explain with fuffi- 
cient facility, when, led by cuftom, they regard its ex¬ 
iftence as well eftablithed. Thefe difficulties have caufed 
feveral chemifts, and in particular Macquer, to believe, 
that fire is nothing elfe but light, and that heat is a mo¬ 
dification of bodies arifing from the motion and collifion 
of their particles. This opinion no longer exifts among 
philofophers who cultivate the fcience of chemiftry. To 
form an adequate notion of the different theories pro- 
pofed for fome years paft, refpeCting fire, we muft not con¬ 
fine attention to general pofitions. The ideas we fhould 
deliver, would be as vague as the fubjeCt itfelf. The only 
method of attaining exaCtnefs, is to divide the fubjeCt, 
and examine its operations of light and heat, attributed 
to combined fire, formerly called phlogifton. 
OF LIGHT. 
The phyfical properties of light are, elaflicity, weight, 
reflection, and tranfparency. It would be difficult to- 
make experiments on this part of natural philofophy, 
without entering into difquifitions which do not belong to- 
this article. Such are in general the properties of light 
as free, or as emanating from the fun and the fixed ftars. 
Let it be our province to confider it chemically, or in the 
mode in which it aCls upon bodies, forms combinations, 
decompofitions, &c. 
Effetts of light upon acids. —-If concentrated nitric acid 
be poured into a decanter, with a ftopper of cryftal, and 
expofed to the fun’s rays, in a certain time the decanter 
will be full of red vapours, and become nitrous. If 
oxygenated muriatic acid be expofed to the light, oxygen 
gas is produced. For this purpofe, the muriatic oxy¬ 
genated acid is to be poured into a decanter; adapt 
thereto a glafs tube, which is to pals under the bell-glafs 
of an alembic, or jar of the pneumatic machine, hereaf¬ 
ter to be deferibed; care muft be taken that the tube do 
not touch the acid. The light ftriking direftly upon the 
liquor, decompofes this acid; one part of the oxygen 
oxyds the manganefe which was employed in oxygena¬ 
ting the muriatic acid, and the other is difengaged in 
the ftate of oxygen gas: thus w r e readily obtain common 
muriatic acid. 
Effcfis on metallic oxyds. — If red oxyd of mercury be 
expofed to the rays of the fun, the oxyd will be decom- 
pofed j the oxygen it contains, which makes it an oxyd,. 
will be difengaged, and the metallic oxyd will recover its. 
natural ftate. Metallic falts may alfo ferve as proofs to 
Ihew the adtion of light. Muriat of fiiver is thus prepa¬ 
red : Take nitrat of fiiver in folution j add muriatic 
acid thereto; it attacks the oxyd of fiiver, and forms an 
indifl'oluble fait, which is muriat of fiiver. Expofe this 
muriat to the light, on a piece of glafs which has black 
paper parted underneath it: in a few hours it will be feen 
that the light has a died only on the furface j the fait is 
reduced to the metallic ftate ; but where the light has 
not penetrated there is no decompofition. 
EffeSls on vegetation. —Light is hurtful to the growth, 
of feeds. Put the feeds of creffes on a bit of cork wrap¬ 
ped up in blotting-paper j leave it to float on the water 5. 
if deprived of light, you will fee how quickly they will 
thoot up, compared with Inch to which light is admitted. 
The more light they are expofed to, the more tardy are. 
the feeds in growing up ; fo that the brighteft light, that 
of the tun, is always the moft hurtful to them. 
Light prefents another very curious phenomenon : 
Place a certain number of leaves in a bell-glafs full of 
water ; reverfe it, and expofe it to. the afition of caloric •, 
a gas will be difengaged from the leaves, which, befides- 
jnany other properties, will have that of being improper 
for 
