ATMOS 
the identity of fire and light, though he feems inclined to 
believe it.” M. de Luc compares elementary fire to a con¬ 
tinuous fluid, whofe parts are condenfed by being mutu¬ 
ally comprelled. He denies that fire and light are the 
fame ; and maintains, that the latter is incapable, by it- 
felf, of felting fire to bodies, though it does fo by putting 
in motion the igneous fluid they contain; and that it aits 
with more force near the earth than at a diftance from its 
Airfare, by reafon of this fluid, which he calls aq heavy 
and ctaflic one, being more condenfed there than at a great¬ 
er height. 
M. Saufiure, in treating of this fubjeft in his account 
of the Alps, does not confider fire as a fluid fo free and 
detached as to be able either to afeend with rapidity by its 
fpecific levity, or to condenfe itfelf fenfibly by its proper 
weight. He fuppofes it to be united to bodies by fo drift 
an affinity, that all its motions are determined, or at lead 
powerfully modified, by that affinity. As foon therefore 
as fire, difengaged by combudion, or by any other caufe, 
endeavours to dift'ufe itfelf, all the bodies that come with¬ 
in the fpliere of its activity endeavour to attraft it; and 
they abforb fuch quantities of it as are in the direft ratio 
of their affinities with it, or in the inverfe ratio of what 
is neceflary for their equilibrium with the furrounding bo¬ 
dies. Now it does not appear, that in this didribution the . 
lituation of places, with regard to the horizon, has any 
other influence than what they receive from the different 
currents produced by the dilatation of the air, and by the 
levity which that dilatation produces. The afeent of flame, 
fmoke, See. or of air heated in any way, perfuaded the an¬ 
cients that Are is poflefled of abfolute levity, by which it 
had a tendency to mount upwards. “ But tliefe effefts 
(fays he) are owing either to the levity of the fluid w hich 
conditutes flame, or to that of air dilated by heat; and 
not to the levity of the igneous fluid. 1 am, however, 
Aifficiently convinced, that this fluid is incomparably light¬ 
er than air, though I do not believe that it poflefles the 
power of afeending in our atmofphere by Virtue of its le¬ 
vity alone. . 
“ The celebrated Bouguer has demondrated, by prin¬ 
ciples the mod Ample, and mod univerfally adopted, that 
it is not neceflary, in order to account for the diminution 
of heat on mountains, to have recourfe to hypothefes that 
are at bed doubtful. The following is his explanation of 
what was felt on the mountains of Peru : It was proper, 
in order to explain this fubjeft, to indd on the diort dura¬ 
tion of the fun’s rays, which cannot ftrike the diderent 
Tides of mountains but for a few hours, and even this not 
always. A horizontal plain, when the fun is clear, is ex¬ 
pofed at mid-day to the perpendicular and undiminidied 
action of thefe rays, while they fall but obliquely on a 
plain not much inclined, or on the ddes of a high pile of 
fieep rocks. But let us conceive for a moment an infula- 
ted point, half the height of the atmofphere, at a didance 
from all mountains, as well as from the clouds which float 
in the air. The more a medium is tranfparent, the lefs 
heat it ought to receive by the immediate aftion of the 
fun. The free paflage which a very tranfparent body al¬ 
lows to the rays of light, Ihews that its fmall particles are 
hardly touched by them. Indeed, what impreflion could 
they make on it, when they pafs through almofl without 
obdruftion ? Light, when it condds of parallel rays, does 
not by paffing through a foot of free atmofpheric air, near 
the earth, lofe an hundred thoufandth part of its force. 
From this we may judge how few rays are weakened, or 
can aft on this fluid, in their paflage through-a dratum of 
the diameter not of an inch or a line, but of a particle. 
Yet the fubtsilty and tranfparency are dill greater at great 
heights, as was obvious on the Cordilleras, when we look- 
cdjat diftant objefts. Ladly, the groffer air is heated below 
by the contaft or neighbourhood of bodies of greater den- 
lity than itfelf, which it furrounds, and on which it reds; 
and the heat may be communicated by little and little to a 
certain diftance. The inferior parts of the atmofphere by 
this means contraft daily a very confiderable degree of 
. YOL.lI, 
P H E R E. 477 
heat, and may receive it in proportion to its denfity or 
bulk. But it is evident, that the fame thing cannot hap¬ 
pen at the diftance of a league and a half or two leagues 
above the furface of the earth, although the light there 
may be fomething more aftive. The air and the wind 
therefore mud at this height be extremely cold, and cold¬ 
er in proportion to the elevation. Befides, the heat ne¬ 
ceflary to life is not merely that which we receive every 
inftant from the fun. The momentary degree of this heat 
correfponds to a very fmall part of that which all the bo¬ 
dies around us have imbibed, and by which ours is chiefly 
regulated. The aftion of the fun only ferves to maintain 
nearly in the fame date the Ann of the total heat, by re¬ 
pairing through the day the lofs it fudains through the 
night, and at all times. If the addition be greater than 
the lofs, the total heat will increafe, as it happens in dim¬ 
mer, and it will continue to accumulate in a certain de¬ 
gree ; but, for the reafons already given, this accumula¬ 
tion cannot be very great on the top of a mountain, where 
the Aimmit, which riles high, is never of great bulk. 1 ne 
lowed date of the thermometer in every place is always in 
proportion to the heat acquired by the foil; and, that heat 
being very fmall on the top of a mountain, the quantity 
added to it by the fun during the day mud be compara¬ 
tively greater; and the accumulated heat will be more in 
a condition to receive increafe in proportion to its didance 
from the degree which it cannot pafs. 
“ Another particular obfervable on all the high places of 
the Cordilleras, and w hich depends on the fame caufe, is, 
that when we leave the diade, and expofe ourfelves to the 
fun-dune, we feel a much greater difference than we do 
here in our fine days when the weather is temperate. Eve¬ 
ry thing contributes at Qmto to make the fun exceedingly 
powerful: a Angle dep from an expofed place to the (hade 
gives the fenfation of cold : this would not be the cafe if 
the quantity of heat acquired by the foil were more confi¬ 
derable. We now alfo fee why the fame thermometer, 
put fird into the diade and then in the fun, does not un¬ 
dergo the fame changes at all times and in all places. In 
the morning, upon Picliincha, this indrument is generally 
a few degrees below the freezing point, which may be reck¬ 
oned the natural temperature of the place ; but, when du¬ 
ring the day we expofe it to the fun, it is eafy to imagine 
that the efteft mud be great, and much more than double 
in whatever way it is meafured.” 
This theory is adopted by M. Saufliire, who adds the 
following faft to prove that the aftion of the fun’s rays, con- 
'fidered abftraftedly and independent of any extrinlic fource 
of cold, is as great on mountains as on plains, viz. that the 
power of burning lenfes and mirrors is the fame at all' 
heights. To afeertain this faft, he procured a burning- 
glafs fo weak that at Geneva it would juft fet Are to tin¬ 
der. This he carried, with forne of the fame tinder, to» 
the top of mount Saleve (a height of 3000 feet) ; where 
it not only produced the fame effeft, but apparently with 
greater facility than on the plain. Being perfuaded then, 
that the principal lource of cold on the tops of high moun¬ 
tains is their being perpetually furrounded with an atmol- 
phere which cannot be much heated either by the rays of 
the Am on account of its tranfparency, or by the refl'eftion 
of them from the earth by reafon of its didance, he wifhed 
to know, whether the direft fohir rays on the top of a high 
mountain had the fame power as on the plain, while the 
body on which they afted was placed in fuch a manner as 
to be unaffefted by the furrounding air. For this pur- 
pofe he indituted a fet of experiments, from which he drew 
the following conclufions, viz. that a difference of 777 toi- 
fes in height, diminiffies the heat which the rays of the fun 
are able to communicate to a body expofed to the exter¬ 
nal air, 14° of the thermometer; that it diminidies the 
heat of a body partially expofed, only 6°; and that it 
augments by i° the heat of a third body completely de¬ 
fended from the air. 
Hence it appears that the atmofphere, though fo eflen- 
tially neceflary to the fiipport of fire, is foruehow or other 
6 F the 
