218 H E 
them, namely, thofe of four and ten inches wide, the 
lid was not foldered, but could be adapted or removed 
at pieafure ; and, there being no occafion for an aper¬ 
ture, the (tern of the thermometer was palled through a 
fhort pipe. One fide was conftantly kept clean and bright, 
the oppofite one was covered with writing-paper palled 
to it, or was painted over with a coat ot lamp-black 
mixed up with as much fize as would make it take a 
body. The other tides, being allotted for mifcellaneous 
experiments, were, according as the cafe required, coat¬ 
ed indifferently with tinfoil, or coloured paper, or dif¬ 
ferent pigments, or had the nature of their furface 
changed by mechanical or chemical agents. Thefe canil- 
ters were placed on Itools, fo contrived as to have their 
centres in the horizontal axis of the reflebtor. 
The chief inftrument in the apparatus is differen¬ 
tial thermometer ; which confifts of a tube bent into the 
form of the letter U, each extremity being clofed by a 
hollow ball, and fulphuric acid tinged with carmine be¬ 
ing introduced into the tube. Each leg of the inftru- 
ment may be from three to fix inches in height, and the 
balls from two to four inches apart. The lower end of 
the fyphon is cemented into a wooden bafe, lo that the 
two balls may be on a level with the centre of the fpe- 
culum. “ A moment’s attention (lays Mr. Leflie,) to 
the conftrubtion of this inftrument, will fatisfy us that it 
is affebted only by the difference of heat in the eorre- 
fponding balls, and is calculated to rneafure inch dif¬ 
ference with peculiar nicety. As long as both balls are 
of the fame temperature, whatever this may be, the air 
contained in the one will have the fame elafticity as 
that in the other, and confequently the intercluded co¬ 
loured liquor, being thus preffed equally in oppolite di¬ 
rections, muft remain ftationary. But if, for inftance, 
the ball which holds a portion of the liquor be warmer 
than the other, the fuperior elafticity of the confined air 
will drive i,t forwards, and make it rife in the oppofite 
branch above the zero, to an elevation proportional to 
the excefs of elafticity or of heat. The interval be¬ 
tween freezing and boiling water being diftinguifhed into 
an hundred equal parts called centigrade, each of thefe 
fubdivided decimally conftitute the degrees which I em¬ 
ploy, and which, following up the fame fyftem of no¬ 
menclature, would be termed milligrade. With the 
meafures thus ftated, each differential thermometer will 
contain from 100 to 150 degrees. I would obferve, 
however, that filch graduation is feldom pofitively re¬ 
quired, ar.d that, in mod cafes, it is lefs important to 
know the abfolute quantities of heat than their relative 
proportions.”. 
The apparatus being fixed in a clofe room without a 
fire, the canifter w'as placed oppofite to the reflebtor; 
and one of the balls of the thermometer (called, for the 
fake of diftinciion, the focal ball) was placed in the fo¬ 
cus of the reflebtor. Almoft immediately, the coloured 
liquor rofe. When the blackened fide of the canifter 
was eppofed to the reflebtor, the effebt was denoted by 
100P; when a fide.covered with paper, by 98°; when a 
' fide covered with crown glafs, by 90 0 . Any pne of 
thefe experiments, then, clearly, manifefts an accumula¬ 
tion of heat in the focus of the reflector; and, com¬ 
pared together, they indicate a flight alteration of effebt 
attendant on the quality of the fubftance, of which the 
fide of the heated canifter is formed. Prefent to the re¬ 
flector the poliflted fid.c, or any fide covered with tin¬ 
foil, and the liquor in tire differential thermometer finks 
to 12 0 . Here, then, is a manifeft difference of effebt ; 
and, without trefpafling againft the cautionary precepts 
of inductive philofophy, we may fafely fay, that the 
power of emitting heat, fo as again to be concentrated, 
is materially different in glafs and tin; and again, for 
the fake of convenience, glafs, paper, or lamp-black, as 
producing nearly equal effebts, may be put in the fame 
clafs ot fubftances that powerfully emit heat, which can 
be again concentrated. 
A T. 
If heat, however,' can be concentrated at a certain 
point, is one fubftance more qualified than another to 
receive fuch heat?" Cover the focal ball with tinfoil, 
prefent the blackened fide of the canifter to the reflec. 
tor, and the effebt is 20° : prefent the bright fide, and 
the effedt is H : but the correfponding effebts before 
were ioo°, and 12 0 . Hence the power of tin to emit 
heat that can be concentrated, and to receive or abforb 
concentrated heat, is much inferior to the power of 
glafs; and the two circumftances are concomitant, viz. 
the powers of emitting and abforbing heat. If glafs be 
conceived to abforb heat abundantly, then, to be confid¬ 
ent, we muft fay that it refiebts heat fparingly ; and ex¬ 
periment fhows that, if a glafs reflebtor be fubftituted 
for the metal one, the effebt on the thermometer is very 
fmall, and the fame whether the back of the glafs fpe- 
culum be fil vered or not, whether it be fmooth or rough : 
but, if the front of the fpeculum be covered with tin- 
foil, then the effedt on the thermometer is increafed ten¬ 
fold. 
The human mind is fo impatient during the interyal 
which leparates'new and ftrange fabts from acknowledged 
principles, that it foon forms for itfelf a mode of con- 
nebtion, and in fomeway unites effebts withcaufes. To 
account for the curious fabts which now prefent them- 
lelves, it might be fuggefted that heat is a fubtle fluid, 
dreaming like light from heated bodies, and permeating 
all fubftances,. If this fuggeftion were well founded, 
thin fereens between the reflebtor and canifter would 
not materially alter the effedt on the focal ball, and 
would produce the fame alteration in all diftances from 
the canifter : but Mr. Leflie placed a fereen of tinfoil be¬ 
tween the canifter and reflebtor, and the effebt on the 
focal ball was deftroyed : he placed a fereen of plate 
glafs, and the effebt was 20 0 at a certain diftance, but 
lefs when the diftance was increaled; confequently, 
emitted heat does not permeate all bodies equally, nor 
fimilarly to light. If, inftead of glafs, a paper fereen 
be ufed, the effebt is 23 0 . “ What then, (alks Mr. Lef¬ 
lie,) is this calorific and frigorific fluid after which w'e 
are enquiring f It is incapable of permeating folid fub¬ 
ftances. It cannot pafs through tin, nor glafs, nor pa¬ 
per. It is not light, it has no relation to aether, it bears 
no analogy to the fluids real or imaginary, of magne- 
tifm and elebtricity. But why have recotirfe to iiivifi- 
ble agents t 
-— -Quod petis, hie eft. 
It is merely the ambient AIR ! 
“But how fhall we explain the diverfified effebts of 
different fereens > By all of them, the current or pul¬ 
iation of hot or cold air, in its progrefs towards the re¬ 
flebtor, will be completely flopped : and, finee the di- 
rebt abtioii of the canifter is intercepted, the fereen muft 
operate by a fecondary and derivative influence. From 
its pofition it acquires heat or cold, and, in its turn, dis¬ 
plays the fame energy as if it had formed the furface of 
a new canifter of the correfponding temperature. It is 
no valid objebtion, that a fubftance fo thin as paper, be¬ 
ing incapable of containing much heat, is fitted only to 
produce a flight and fugitive effebt. The fereen is en¬ 
abled to maintain its temperature, and confequently to 
continue its abtion, by the perpetual acceilions of heat 
or cold which it receives from the canifter. It hence 
appears, that the quantity of effebt produced upon the 
focal ball when fereens are interpofed, is determined by 
the combined operation of two kindred properties ; their 
aptitude to receive heat, and the power to difeharge it. 
Thus, with paper the effebt is greater than with glafs 5 
becaufe the receptive and the difperfive qualities of the 
former are likewife greater. With tin no perceptible 
impreffion is made, for thofe qualities, it has in a very 
inferior degree ; and though of each taken ftngty the ac¬ 
tion might be difeerned, the effebt of their combined in¬ 
fluence is too minute to be obferved with certainty.” 
4 From 
