L I 
excited by light, who ftates it at the ferenth part of a fe- 
cond, (Hift. Acad. Scienc. 1765.) we may admit an in¬ 
terval of more than 20,000 miles between each particle. 
That light is a real fubltance, notwithftanding the ob¬ 
jections that have been urged againft this hypothecs, 
feems to be eftabliflied by the phenomena of the Bolog- 
nian ftone, and of other fubftances, which poffefs the re¬ 
markable property of imbibing light, of retaining it for 
fome time, and afterwards of emitting it. 
The doitrine of the materiality of light is farther con¬ 
firmed by thole experiments which demonftrate, that the 
colour and inward texture of fome bodies are changed, in 
confequence of their being expofed to the light, The 
firft obfervation of this kind appears to have been made 
by M. Duhamel, who found that the juice of a certain 
fhell-filh in Provence contracted a fine purple colour when 
it was expofed to the light of the fun, and that, the ltronger 
was the light, the more fplendid was the colour. Pieces 
of cloth dipped in this liquor, and expofed to the fun, 
became red, though they were inclofed in glafs ; but they 
acquired none of this colour in the fame expofnre, if they 
were covered with the thinned: plates of metal. It was 
afterwards obferved by Beccarius, (Com. Bonon. vol. iv. 
p. 75.) that a quantity of Luna cornea, expofed to the 
rays of the fun, became of a violet colour, whilft part of 
the fame compofition, covered with black paper, remained 
white. This effeCt was found by M. H. Schulze to de¬ 
pend on the filver that happened to be in it. (Ac. Cas- 
far, vol. i. p. 528, See.) G. Bonzius was thus led to fome 
lubfequent experiments, which feem to prove that various 
colours are confulerably affeded by light, exclufive of 
heat or any thing elfe. By expofing ribbons of different 
colours to the rays of the fun for feveral days in the open 
air, he found that all, except the yellow and light green, 
loft part of their luftre, and were confiderably faded ; but, 
when the fame ribbons were expofed to a much greater de¬ 
gree of heat in a dark room, none of the colours were af- 
iefted, except that a fmall part of their luftre was loft ; 
nor was any fenfible change made in them after remain¬ 
ing for a confiderable time in a room that faced the north. 
By inclofing them in an exhaufted receiver, he found that 
the change was not occafioned by the air ; but no change 
could be produced in them by the light of torches. Bec¬ 
carius alfo found by experiments on paper, and a great 
variety of fubftances, mineral, vegetable, and animal, that 
the light of the fun produced many changes in the inter¬ 
nal ftrufture of bodies, and that thofe fubftances which 
imbibed light were much injured. The ftronger the light, 
and the longer they were expofed to it, the more injury 
they received ; and the injury thus fuftained was found 
to be lafting. 
The mutual aflion between light and other matter is 
productive of numerous phenomena. Thefe conftitute 
two diftinft branches of fcience. The one has for its ob¬ 
ject the i.nveftigation of the phyfical properties of light, 
for which we are principally indebted to Newton, and 
which forms the bads of the fcience of Optics. The 
other is confined to the chemical agency of light, refting 
upon faCts difeovered fince the time of that great genius; 
and for which fee the article Chemistry, vol. iv. p. 153, 
179-181. 
II. I11 general, a very confiderable degree of heat 
is requifite to the emiftion of light from any body ; but 
there are feveral exceptions to this, efpecially in light pro¬ 
ceeding from putrefeent fubftances and phofphorus, toge¬ 
ther with that of luminous animals, and other fimilar ap¬ 
pearances. 
The exhibition of light by living animals is a lingular 
property belonging only to creatures of a fimple ftrufture, 
as infeCts and worms. The Medufa is the only genus of 
worms which has been afeertained to (how light; for the 
account of certain fpecies of Lepas, Murex, and Chama, 
yielding light, does not feem to reft upon any good au¬ 
thority ; and the ftories told by Brugueire and by Flau- 
G H T. 677 
gergues of earth-worms being luminous, are improbable 
in themfelves, even if they were not contrary to common 
obfervation. The examples of luminous fpecies are more 
numerous amongft infeCis than in any other clafs. They 
are to be found in the following genera; Elater, Lampyris, 
Fulgora, Paufus, Scolopendra, and Cancer. See thofe ar¬ 
ticles; particularly LampyrIs, p. 124., 5, of this volume. 
Some writers have attributed the property of (hining 
while alive to certain fijhes ; but, as it would appear, upon 
very queltionable grounds. In a voyage from France to 
Cayenne, Mr. Bajon faw in the fea a number of luminous 
points which (lrone when (truck, and another appearance 
of pale flames, about three feet below the furface of the 
water. He likewife obferved, in the migration of the do¬ 
rado and other fifties, that their bodies were ftudded with 
fimilar luminous points. Upon examination, he difeoveed 
thefe to be minute lpheres which adhered to the furface 
of the fifli : they were mofc probably the fmall fpecies of 
luminous medufa : Bajon’s obfervations therefore proved 
no property of ftiining in the fifli themfelves. 
The firft diftinft account that we meet with of light 
proceeding from putrefeent animal flefli is that which is 
given by Fabricius ab Aquapendente ; who fays, that, 
when three Roman youths, redding at Padua, had bought 
a lamb, and had eaten part of it on Eafter-day, 1562, fe¬ 
veral pieces of the remainder, which they kept till the day 
following, (hone like lo many candles when they were ca- 
fually viewed in the dark. Part of this luminous flefli 
was immediately fent to Aquapendente, who was profef- 
for of anatomy in that city. He obferved, that both the 
lean and the fat of this meat (hone with a vvhitifli kind of 
light; and alfo took notice, that fome pieces of kid’s 
flefli, which had happened to have lain in contaff with it, 
were luminous, as well as the fingers and other parts of 
the bodies of thofe perfons who touched it. Thofe parts, 
he obferved, (hone the molt which were foft to the touch, 
and teemed to be tranfparent in candle-light ; but, where 
the flefh was thick and folid, of where a bone was near 
the outfide, it did not (bine. 
After this appearance, we find no account of any other 
fimilar. to it, before that which was obferved by Bartholin, 
and of which he gives a very pompous defeription in his 
ingenious treadle already quoted. This happened at 
Montpelier in 164.1, when a poor old woman had bought 
a piece of fief, in the market, intending to make life of 
it the day following. But, happening not to be able to 
deep well that night, and her bed and pantry being in the 
fame room, (he obferved fo much light come from the 
flefli, as to illuminate all the place where it hung. A part 
of this luminous flefli was carried as a curiofity to Henry 
Bourbon, duke of Conde, the governor of the place, who 
viewed it for feveral hours with the greateft aftonifliment. 
This light was obferved to be whiti/h ; .and not to cover 
the whole furface of the flefli, but certain parts only, as if 
gems of unequal fplendour had been fcattered over it. 
This flefli was kept till it began to putrefy, when the light 
vaniflied ; which, as fome religious people fancied, it did 
in the form of a crofs. 
It was natural to expedf, that the almoft-univerfal expe¬ 
rimental philofopher Mr. Boyle would try the effect of 
his air-pump upon thefe luminous fubftances. Accord¬ 
ingly, we find that he did not fail to do it; when he pre- 
fently found that the light of rotten wood was extinguiihed 
in vacuo, and revived again on the admiflion of the air, 
even after a long continuance in vacuo; but the extin- 
guifhing of this light was not fo complete immediately 
upon exhaufting the receiver as fome little time afterwards. 
He could not perceive, however, that the light of rotten 
wood was increafed in condenfed air; but this, he ima¬ 
gined, might arife from his not being able to judge very 
well of the degree of light, through fo thick and cloudy % 
glafs vefiel as he then made ufe of; but we find that the 
light of a (hining fifli, which was put into a condenfing- 
engine before the Royal Society in 1668, was rendered 
jpour 
