C H E M 
fuppofe the thermometer dilated by the reflected and 
concentrated light, why ftiouid not that light be ren¬ 
dered vifible by the concentration ? Dr. Hutton con¬ 
ceives, that, fince the peculiar efficacy of this tight, in 
affecting the temperature of bodies, mult depend on its 
greater aptitude to combine with them, this lame difpo- 
lition for combination, will dif'qualify it from appearing 
as light. At the lame time, he leaves to experimental- 
ills to determine how far this fpecies of light, afrer hav¬ 
ing once diTappeared to our ltnfes, is incapable of be¬ 
coming again vifible ; unlefs the temperature of the body 
with which it has combined, be railed to the degree of 
incandel'cence, in which cafe it may pollibiy be emitted 
from the body, as vifible light. Taking it for granted that 
the emanation which affedls the thermometer in thefe 
experiments, is invifible light; we learn, from the fame 
experiments, compared with others made by Marriotte 
and Scheele, that this invifible fpecies of light is much 
abforbed in palling through a.glafs lens, while it is but 
little diminilhed, in being reflected by a metallic furface. 
Dr. Hutton afterwards proceeds to analyfe M. Piftet's 
curious experiment of apparently refledted cold : it will, 
however, be unnecefl'ary to repeat this part of his work ; 
for, although his oblervations on the phenomena of that 
lingular experiment appear to be exceedingly juft, and 
his doubts refpedting the courfe of the radiation to be 
founded on the molt folid grounds, yet he has left the 
lubjedt in a ftate of confelfed uncertainty ; having mere¬ 
ly pointed it out, without having performed any of the 
experiments necefl'ary for its elucidation. In the courfe 
of liis oblervations on this experiment, he is led to pro- 
pole another hypothefis refpedting invifible light. He 
fuppofes that bodies are conftantly radiating invifible 
light, in proportion to their temperatures ; and that this 
invifible light falling upon other bodies, railes their 
temperature, or is refledted by them, according to their 
relpedtive natures. At the lame time, the fenfible heat 
of bodies (the free caloric) is always tending to equili¬ 
brium, according to the laws of its dilfufion, and in con- 
lequence of the mechanical operation of the elaftic fluids 
conveying it from place to place. From the combina¬ 
tion of thefe caufes, the temperature of bodies is always 
tending, on the one hand, to equality; and this perfedl 
equality is, on the other hand, prevented from adtually 
taking place, by the different difpofitions of bodies to 
abforb invifible light, and to have their temperatures af- 
fedted by it. Dr. Hutton applied this theory to explain 
M. Pidtet’s experiment, and at the lame time fuggefts 
an experiment, by which he conceives the truth of his 
own theory may be confirmed, or its fallacy difcovered. 
It may be worth while to add, that M. Prevoft, of Ge¬ 
neva, has publiflied fome fpeculations, extremely analo¬ 
gous, in many rel'pedts, to Dr. Hutton’s views upon this 
tubjedt; with this difference, that what Dr. Hutton calls 
invifible light, M. Prevoft, conformably to Sauflure’s 
opinion, terms chaleur rayonnante. Recherches Pbyfico-Me~ 
chaniquesfur la Chaleur, 1 79Z. 
It is, after all, impoftible for chemifts, in the prefent 
ftate of the fcience, to decide, whether the light emitted 
during combuftion, be derived principally, from the corn- 
buftible body, or from the oxygenous gas ? For it feems 
improper to derive it exclufively from either, when there 
is realon to believe that it enters into the compofition 
of both. It mult be confelfed, indeed, that the evidence 
for light being a conftituent part of oxygenous gas, is 
far from being unexceptionable. The principal argu¬ 
ment is taken from the fingular influence of light, in 
feparating oxygen from its combinations, and making it 
aflume the ftate of oxygenous gas. Light, as we have 
feen, difengages oxygenous gas from the nitric acid, 
from the oxygenated muriatic acid, and from the oxyds 
of feveral metals. But in thefe, and many fimilar in- 
ftances, does the light combine with'the oxygenous gas? 
Or, does it unite with the fubftance from which the oxj'- 
gen is feparated ? There is no fiidt in chemjftry that will 
Vox. IV. No. 188. 
: S T R Y. • 181 
enable us to decide between thefe two hypothefes; and 
thole chemifts who, with Fourcroy, regard light and ca¬ 
loric as two fubftances eflentially different, have abfo- 
lutely no realon for inclining to the one hypothefis, in 
preference to the other. The exiltence ot light, as a 
conftituent part of combuftible bodies, is proved in the 
molt latisfactory manner, by the experiments of Deiman, 
Pacts, and Van Trootfwyck. Thefe chemifts expofed a 
mixture of lulphur and zink to a high temperature, with¬ 
out any fubftance being prefent, from which they could 
derive oxygen. At the inllant when the lulphur and 
zink formed a fulphure, there was a vivid emiffion of 
light; and, when the materials were afterwards examin¬ 
ed, it was found that no oxydation had taken place. 
This experiment fucceeds with other metals, befides zink; 
fuch as copper, tin, lead, and faintly with iron. It has 
been tried with equal fuccefs in vacuo, above mercury, 
and in hydrogenous, azotic, and carbonic acid, gafes. 
It has been for a long time a defideratum in chemif- 
try, to pollels a method of meafuring the quantity of 
light emitted from bodies in the ftate of combination. 
Count Rumford has publilhed, in the Pliilofophical 
Tranlaftions for 1794, the delcription of a very ilmple 
inftrument, which he has contrived for this purpofe. 
The following are the principle refults of his experi¬ 
ments with this inftrument, which is called a photometer. 
The tranl’parency of air is fo great, that the diminu¬ 
tion which light buffers in pafiing through fixteen or eigh¬ 
teen feet of air, is not perceptible with this inftrument. 
Somewhat lels than z-ioths of a beam of light are loft in 
palling through a pane of fine, clear, transparent, well- 
polifhed, glafs, fuch as is commonly made ufe of in the 
conltruftion of looking-glafl’es ; fomewhat more than 
3-ioths are loft in palling through two panes of fuch 
glafs; and about 12-iooths, in palling through a pane 
of thin colourlefs window-glafs. The light fell perpen¬ 
dicularly upon the glafs in thefe experiments ; but the 
lofs was very little greater in oblique incidences, pro¬ 
vided the angle of incidence did not exceed 4.0° or 50°. 
Somewhat more than i-3d of a beam of light was loft by 
reflection from a yery excellent glafs mirror; and about 
46-iooths, from a common looking-glafs. The angle of 
incidence was 45 0 , and the rel’ult was not fenfibly af¬ 
fected by increafing the angle to 85°. Count Rumford 
has collected, in the following table, the refults of his 
experiments on combuftion. It is to be obferved, that 
each of the under-mentioned articles is luppofed to af¬ 
ford the fame quantity of light; the table denoting how 
much of each mult be employed for that purpofe. 
Equal parts 
in weight. 
Bees wax. A good wax candle, kept well fnuff- 
ed, and burning with a Gear bright 
flame ... 10® 
Tallow, A good tallow candle, kept well 
fnuffed, and burning with a bright 
flame . . . . .101 
The fame tallow candle, burning 
very dim, for want of lnufling . 229 
Olive oil. Burned in Argand’s lamp . . . .11® 
The fame burned in a common lamp, 
with a clear bright flame, without 
fmoke.129 
Rape oil. Burned in the fame manner. . . . 125 
Lintieed oil. Likewife burned in the lame manner 100 
Count Rumford concludes his paper with an account 
of fome experiments on the tranfparency of flame ; from 
which it appears that light lullains no fenfible diminu¬ 
tion in confequence of pafiing l’ucceffively through the 
flames of feveral candles. 
OF CALORIC. 
Caloric, as we have feen above, is a fubftance which 
produces the ienlation of heat. The caloric of the 
French chemifts is the phlogifton of Stahl and Priejf- 
3 % 5 
