C H E M I 
niae, or volatile-alkali, the acid of nitre, and animal 
fubitances. 
A cOinbuftible body which has burnt in atmofpheric 
air, and ablbrbed all the vital air to which it is capable 
of uniting, can burn no longer in a frefh quantity of air: 
it has become incombuftible, and frequently laline. A 
body burning in atmofpheric air never abforbs complete¬ 
ly the twenty-f'even hundredth part of vital air it con¬ 
tains. To make a perfect analyfis of the atmofpheric 
air, and dived: it entirely of this fluid, combuftible bodies 
mult be burnt in it repeated times. The portion of air 
thus abforbedby combultible bodies, called above vital 
air, is alfo named oxygen gas. The former name is de¬ 
rived from its being the only elaftic fluid capable of fup- 
porting life: the latter is given it, becaule many bodies 
on abiorbing it are rendered acid. 
Combuftion, then, confifts in the fixation and abforp- 
tion of vital air by combuftible bodies, and the decom- 
pofition of the atmofpheric air by thefe bodies. As the 
vital air only ferves to liipport combuftion, it is eaf'y to 
conceive, that a very combuftible body, capable of ab- 
lorbing the whole of the vital air, may be employed to 
determine the proportion of the two atmofpheric fluids : 
thus phofphorus is now ufed for thepurpofeof eudiome- 
try, or to difcover the purity of the atmofphere, that is to 
fay, the proportion of this vital air which it contains. 
As vital air is a gas, and many combuftible bodies, when 
they abforb it, render it fixed, and caui'e it to affume the 
folid form, the vital air, when it is thus precipitated, lofes 
the caloric, which held it in a fbrte of folution, and gave 
it the elailic fluid form: hence the origin of the caloric 
dilengaged, or of the heltt produced, during combuftion. 
Combuftible bodies differ from each other, firft, In the 
rapidity with which they abforb oxygen; 2diy, In the 
quantity of it they abforb; 3 dly, In the proportion of 
caloric which they difengage f rom the oxygen abforbed • 
and, by confequence, athly, In the greater or lei's degree 
of folidity of the oxygen they, contain after being burnt. 
Burnt bodies, then, may be defined to be bodies combin¬ 
ed with oxygen : accordingly they are termed oxygenat¬ 
ed or oxydated fubftances : and as the greater number 
of known bodies are either combuftible, or already burnt, 
we may be allowed to fufpeCt, that many incombuftible 
natural bodies, with the compofition of which we are 
unacquainted, are incombuftible folely from being iatu- 
rated with oxygen. With regard to lame in this predi¬ 
cament, this conjecture has already been verified. 
From feveral of the preceding axioms, it follows, that, 
when we burn a combuftible body 111 order to procure 
heat, as we do to mitigate the rigours of winter, we ob¬ 
tain at leaf! the greater part of tile caloric from the airit- 
feifVwith which it was combined. We may even afl’ert 
that the colder the air, the more heat is derived from it; 
becaufe, when the atmofphere is extremely cold, more 
air paffes into the fire in a given bulk. Indeed, it is well 
known, that the fire in our grates is much more fcorch- 
ing, and burns much more brifkly, when the air fuddenly 
becomes cold; and the art of increafmg combuftion by 
means of condenl'ed air thrown from a pair of bellows on 
wood already heated, is founded on this principle. 
Combuftion, therefore is not confined to the decompofi- 
tion of atmofpheric air by abforbing one of its principles ; 
for it alio decompofes the vital air, by.abforbing, fixing, 
and rendering more or lei's f'oiid, in the combuftible bo¬ 
dy, the oxygen, or bale of the vital air, and difengaging 
the. folvent of this bale,.caloric, in greater or lefs quantity. 
There isanother interelling phenomenon in combuftion, 
which modern chemlitry is able to explain : that of the 
dii'engagement of light, or the production of flame. It 
is demonftrated, that the greater part of the light which 
conftitutes flame, is contained in the vital air, of which 
it is one of the principles: for, ift, Combuftible bodies 
afford much more flame when they burn in vital air a- 
lone, than in atmofpheric'air: 2dly, There are colnbuf- 
tible bodies which do not burn vvicn flame except in vi- 
s T R Y. 155 
tal air: 3clly, To difengage the oxygen from bodies 
which contain it, and com ei t it into vital air, it is not 
fuflicient to diffolve it in a greater or lefs quantity of ca¬ 
loric, but it is neceffary at the fame time to add light: 
4-thly, There are burnt bodies which lofe their oxygen 
on the conlaft of light alone.: in this fenfe we mult un¬ 
derhand the property of unburning and decombuition, 
mentioned above, as a character ift ic of light. Vital air, 
therefore, is to be coniidered as a compound of a iblidifi- 
able, ponderous, acidifying, b ile, oxygen, diffolved in' 
two menltrua, caloric, and light, which of thernfelves are 
extremely attenuate, highly elaftic, and deflitute of af~ 
fignable weight. Combuftion confifts in a more or lefs 
complete precipitation of the oxygen of thefe two men¬ 
ltrua. Thus, a combuftible body, in burning, dif’en¬ 
gages from vital air, not only caloric, but alio light; and 
every combuftible body difengages a different quantity 
of light from the vital air, as it does of caloric. It is 
probable that there are combuftible bodies which fepa- 
rate from vital air more light than caloric, while others 
difengage from it more caloric than light. The oxygen 
fixed in burnt combuftible bodies remains more or lefs de¬ 
prived of light and caloric; and the denfity or folidity it 
acquires in, the procefs, is one of the caufes to which is 
owing the greater or lefs facility experienced in fepara- 
ting the oxygen from burnt bodies in the form of vital 
air. For this, fome require more caloric than light; 
others more light than caloric. It is eafy to perceive, af¬ 
ter what has been laid, that to feparate the oxygen from 
a burnt body, is to perform an operation the reverie cf 
combuftion. We have no word in our language to ex- 
profs this Operation. It would not be improper to fay, 
that we unburn, that we difoxydate, the body : hence the 
terms of unburning and difoxydation. 
Befide the greater or lefs force with which oxygen is 
retained in combuftible bodies, according to .its being 
combined with them inaliate of greater or lefs lblidity, 
and its having loft a greater or lefs portion of its folvents, 
caloric and light; it adheres to them by its attraction, 
its particular affinity to each. A confiderable number 
of thefe affinities of oxygen for different fubftances are al¬ 
ready known, and the degrees of f'ome cf them have been 
afeertained. It is from theftlegrees of thefe affinities, 
that we are frequently enabled to transfer oxygen from a 
burnt body to a combultible one. I11 this procefs a com¬ 
buftion takes place, lb much the more imperceptibly, or 
tacitly, as it were, according as the oxygen is more lblid 
in the burnt body, and more iimilar in denfity to the bo¬ 
dy which abforbs it, or into which it panes. But this 
kind of combuftion fometimes takes place with a vivid 
heat and flame: which phenomena occur, whenever the 
body which is to receive the oxygen mult contain it in a 
more folid form than that from which it is extracted. 
Thus iron, zink, antimony, arlenic, tec. burn with flame, 
when heated with oxyd of mercury, from which they 
attraft the oxygen, to contain it i.i a more f'oiid form- 
From thefe confederations we may deduce the nature and' 
caufes of the obstacle which air oppofes to evaporation, 
the ebullition of liquids, fublimation, &c. the folution. 
of water in air, and the hygrornetrical ltate of the atmof- 
phere: the effloreicence and deiiquefcence of laline bo¬ 
dies : aqueous meteors: experiments made at different 
heights of the atmofphere, and in a vacuum : the compa¬ 
rative nature of combuftible .bodies: the increaie of 
weight and change of nature in thefe bodies after com¬ 
buftion : artificial heat and flame; the theory of furna¬ 
ces: the different eudiometrical proceffes: the refpira- 
tion of different animals: the mephitilin occafioned by 
combuftion and refpiration: the diminution, increaie, 
and lupport, of animal heat: tranfpiration from the fkin 
and lungs, &c. 
THE NATURE AND ACTION OF WATER. 
Water exifts in three different flares : that of a folid, 
which is ice; that of a'liquid, its molt common form, 
and 
