H E 
From the preceding ftatements it appears, that thefe 
fcreens aft merely as fecond canifters : but, to put the 
matter beyond all doubt, the author made the following 
extremely ingenious experiment : “ Seleft two panes of 
crown-glafs as fiat and fmooth as pofiible, and coat one 
fide of each with tinfoil, by means of a little gum-water. 
Thus prepared, and the apparatus put in order, join 
thofe panes together with their tin furfaces in contaft, 
and attach them to the frame of the fcreen ; the focal 
ball will receive an impreflion equal to about i8 Q . In¬ 
vert the panes of glafs, placing them with the tin coat¬ 
ings outmoft : the liquor of the differential thermome¬ 
ter will now fink back again to the beginning of the 
fcale. Such is the experimentum crucis. It eftablifhes 
beautifully, and, I think, beyond the ppwer of contra¬ 
diction, the fimple theory to which we have been led by 
a clofe train of induction. In both cafes the obftacle 
pref’ented, or the compound fcreen, is abfolutely the 
fame. If the eftefts in the focus of the refleftor were 
produced by fome fubtle emanation capable of permeat¬ 
ing folid fubftances, how could fuch a fingular contraft 
obtain?”—It feents impoflible to elude the force of this 
argument; but to.confirm and illuftrate the fame con- 
clufion, further experiments were made, which fully 
eitublifiled the conclufion thus deduced. 
From the concentration and accumulation of heat, 
.then, on the focal point of the refleftor, and from the 
ftoppage of heat by the interpolition of fcreens, it ap¬ 
pears that a flow of heated matter proceeds from the 
heated canifter. Would fuch a flow, under, fuch cir- 
cumftances, take place in non-elaftic fluids,—in water, 
for inftance ? The foliowing experiment gives.a deci- 
iive anfwer to this queftion : Place the apparatus within 
a large tub, and fecure each feparate part in its proper 
pofition. Fill the tub with cold water, fo as to cover 
the whole, except a funnel foldered to the mouth of the 
canifter. Things being thus difpofed, pour boiling wa¬ 
ter into the canifter ; and whatever furface fronts the 
refleftor, the differential thermometer will not be at all • 
affefted Thus, by means of thefe fkilfully-condufted 
experiments, we arrive at two very important fafts; 
firft, that, in air andelaftic fluids, bodies difcharge part 
of their heat with powers and energies that depend on 
the quality of the heated furface; and, fecondly, that 
in non-elaftic fluids, this peculiar mode of difcharging 
heat does not take place. This property, or mode of 
difcharging heat, then, feems intimately to depend.on 
the elafticity of the medium through which it is con¬ 
veyed ; and therefore may not heat be conveyed by the 
pulfations of the elaftic medium, and, when the medium 
is air, with the fame velocity as found ? 
Mr. Leflie, after the moft mature contemplation of 
the refults of his experiments, prefumes that heat is 
matter, a gafeous fluid of great tenuity, capable of che¬ 
mically combining with the particles of other bodies. 
He fltows, with great ftrength of argument, that it'can¬ 
not be a mere effeft produced by vibration or internal 
motion ; and that, by mechanical addition, or Ample 
aggregation, it could not effeft thofe dilatations which 
it is tound to produce. Again, he fuppofes that, as ac¬ 
tual contact never takes place, the interval which fepa- 
rates the furface of a body from the medium in. which 
it is placed is greater, or lefs, according as the chemi¬ 
cal attraction between the particles of the body and the 
medium is lets or greater. Hence the boundary be¬ 
tween air and glafs is lefs deep than the boundary be¬ 
tween air and metal. Now, when the particles of air 
approach a heated furface, they receive a fudden accef- 
fion of heat, and confequent expanfion, which caufes 
them quickly to recede from the heated furface, in the 
form of initial waves-: the lefs, then, the interval be¬ 
tween the particles of air and the furface, the more fre¬ 
quently will the contact be renewed, and the more 
quickly will one hot pulfation follow another. Meat,- 
therefore, will be propagated by means of a number of 
A T. 27 9 
concentric fpherical (hells of air; eenfequently, the 
diftance from the heated furface being increafed, the in- 
tenfity of the heat declines as the fhell augments ; and, 
according to this doCtrine, all heat in the fame elaftic 
medium will be tranfmitted with equal velocity, and the 
quantity conveyed will depend on the interval between 
confecutive pulfes. If the circumfiauce of the conti¬ 
guity of the particles of air to thofe of the heated fur¬ 
face were the real caufe of the difference in the quan¬ 
tity of emitted heat, then, if by any means the conti¬ 
guity between the particles of air and of metal could 
be increafed, more heat ought to be emitted ; and fuch 
is the faft : for Mr. Leflie furrowed or ftriated the fur¬ 
face of the tinfide of the canifter, and the differential 
thermometer immediately indicated an augmented effeft. 
A variety of experiments were made to afcertain the 
effeft: of fubftratums over the metal and glafs fides ot 
the canifter. The thinneft metal covering over the glafs 
fide of the canifter reduces the pulfatory power of that 
fide to the power of a metal fide : but the cafe is dif¬ 
ferent when pellicles of ifinglafs are fpread over a me¬ 
tal fide: the pulfatory power increafes with the thick- 
nefs of the pellicle; and when the effeft of the black¬ 
ened fide of the canifter is reckoned ioo°, the effeft of 
a thin coat of jelly applied to the bright metal fide of 
the canifter was 38°, and that of a coat four times as 
thick, 54 0 . To make experiments correlative to thefe, 
the furface of the tin refleCfor ought to be covered with 
thin pellicles of ifinglafs : for then, the recipient power 
of the refleftor being augmented, its refleftive power 
ought to be diminiftied ; and .experiments adapted for 
the purppfe proved this to be the cafe. 
When the fpherical aerial (hells, charged with heat, 
come into contact with the refleftor, part of the heat is 
abforbed by the latter ; the quantity abforbed depend¬ 
ing on the nature of the refleftor itfelf. The reft, not 
feparately, but combined with air, is reflected to the 
focus. When light is reflected, the angles of incidence 
and reflection are equal ; and if light be an impinging 
fluid, this exaft equality of incidence and reflection 
may be explained by the amazing velocity with which 
light impinges. If the velocity were lefs, the equality 
of reflection would not take place ; and heat, therefore, 
impinging only with the velocity with which found is 
propagated, (1142 feet in a fecond,) is imperfeftly re¬ 
flected : confequently, there muft be an aberration in 
its focus; and the experiments before us clearly (how 
that fuch is the faft. 
Mr. Leflie introduces many ftrong arguments to prove, 
that light and heat are the fame fluid ; that light is no¬ 
thing more than heat in a (late of combination with bo¬ 
dies, and that the chief diftinftion confifts in the velo¬ 
city with which the former is endued. He further ob- 
ferves, that if a body be expofed to the fun’s-rays, it 
■will, in every poftible cafe, be found to indicate a mea- 
fure of heat exaftly proportioned to the quantity of 
light which it has abforbed ; a faft which, if well found¬ 
ed, furnifbes a logical inference that light and heat are 
identical. 
The cooling or refrigerating principle is the next fub- 
jeft of difcullion. Metal bodies conduct heat more ra¬ 
pidly than earthen, or vitreous, or wooden, bodies ; and 
from this fa'ft it has been inferred, that heated metal 
bodies cool fafter in air than vitreous bodies : but the 
focal experiments firft related (how, that the latter bo¬ 
dies emit a portion of heat, which can again be concen¬ 
trated, more copiouily than the former. There is, there¬ 
fore, one caufe why a vitreous ought to cool fafter than 
a metal body. But in thefe refearches, it is proper to 
be governed by experiment.—“I11 a clofe room,'with¬ 
out a fire, (fays Mr.' Leflie,) -having placed a thin hol¬ 
low globe of pianifhed tin,- 1 four inches in diameter, and 
with a narrow neck, on a (lender metal frame or ltool, 
and refting againft the (harp edge, I filled it with warm 
water, arid infer fed'a thermometer. The air of the 
room 
