180 CHE M 
for combuflion. If, on the contrary, the fame appara¬ 
tus be exp tied to the fun's rays, a gas will be produced 
which has the property of being fit for combnftion: which 
thews clearly that the produmon'.of this fluid arifes from 
the influence of the light upon the leaves. 
Light changes the colour of vegetable fubftances ; de-' 
prived of light, they become pale and infipid, and are 
what we call etiolated. Plants grow very well in perfect 
darknefs, and even increafe falter than in the fun, pro¬ 
vided the air which furrounds them be refpirable. The 
mode the gardeners ule for the etiolation or bleaching of 
endive, celery, lettuce, &c. is well known ; when come 
to a certain height, they bind them round, or earth them 
up, and thus deprive them cf all contaft with the light. 
Maturity and the refinous principle are properties which 
depend thereon. An obfervation made by M. Humboldt, 
might lead to uleful inquiries on the fubjedt of etiolation, 
lie found that feveral plants of the clals cryptogamia, 
(fee Botany, vol. iii. p. 279-) fuch as the lichen verti- 
cellatus, pufli out green Italics, though pi iced in mines, 
where the light has no accefs to'them; and that feveral 
of the gramma, the poa annua and trefolium arvenfe in 
particular, have green leaves, when growing in the gal¬ 
leries of certain mines, at the depth of lixty toifes be¬ 
low the furface of the earth. Examining the air in 
thefe fituatio.ns, he found that it was mingled with hy¬ 
drogenous gas. It would feem, from this cafual ob- 
fervation, that the prefence of hydrogenous gas compen- 
fates, to a certain extent, in vegetation, for the abfence 
of light. M. Ingenhoufz has alfo remarked, that when 
hydrogenous gas is mingled with the air in which vege-= 
tables grow, the verdure of the latter acquires a much 
deeper (hade. M. Humboldt alfo mentions, that he has 
difeevered another fpecies of lichen in the mines at Ma- 
rienburg, which had ramifications of a bright green co¬ 
lour. He alfo found, that the cheiranthus incanus, and 
cheiranthus cheiri, placed in certain circumftances, pufli 
out green leaves, without the prefence of light. He im¬ 
putes thefe lingular phenomena to the hydrogenous gas 
uniting with the excefs of oxygen, which he fitppofes to 
be prefect in etiolated plants, and to be the immediate 
caufe of their etiolation. See Journal de Phjfique, part 2d, 
for 1793. 
The more plants are expofed to the folar rays, the 
more colour they acquire. Such, therefore, is the ori¬ 
gin of thofe colouring matters, of fo much value for 
their livelinefs and durability, which many of the ealtern 
nations extract from woods, bark, and roots, &c. and 
which the utmoft induflry of the European dyers has not 
fkcceeded in imitating. But colour is not tlie only pro¬ 
perty that is obtained by vegetables from the contadl of 
the rays of light. Take, odour, and combullibility, are 
likewife derived from the fame fource. Light contributes 
greatly to the maturity of fruits, and is the caufe why, ur.* 
der the burning fun of South America, vegetables are in 
general more odoriferous, of a ftronger ffate, and more 
abounding with refin. From the fame caufe it happens, 
that hot climates feem tu be the native country of per¬ 
fumes, ftrong fmelling fruits, dying woods, and refills, of 
various kinds. Laftly, the aflion of light is fo powerful 
on the organifm of vegetables, as to caufe them to pour 
forth torrents of vital, or pure air, from the fuperior 
iurtace of their leaves into the atmofphere, while expofed 
to the fun-fhine; whereas, on the contrary, when in the 
fhade they exhale nothing but a noxious fluid, or true 
acid, (miliar to that obtained from chalk. This im¬ 
portant difeovery, due to Dr. Prieftley, and more mi¬ 
nutely inquired into by M. Ingenhoufz, (hews, in a link¬ 
ing manner, the influence of light on vegetation. 
W e fee, likewife, that the influence of light upon living 
animals has a very ftnfible effedl: earth-worms and wood- 
lice are whitifh; birds of night, and moths, are diftin- 
guifhed from birds who feek the light, and from butter- 
■flies, by their pale ini perfect colours : there is a difference 
alfo between thofe of the north and thole of the foutb. 
S T R Y. 
Animals accuflomed to the light, on being deprived cf 
it, fall lick, and even die. Ira man be long {hut in a 
dark dungeon, puttules filled,with watery matter, arile 
on the (kin, and he becomes dropfical. 
Dr. Hutton, of Edinburgh, has lately pointed out a new 
principle in the adlion of light, confiaered as a power in¬ 
fluencing the temperature of bodies ; and has applied this 
principle to the explanation of various phenomena. Dif- 
fertation on the Philcfophy of Light, Heat, and Fire, 1794. 
The following are his (peculations upon that fubjedl: he 
obferves, that the efficacy of the different fpecies cf light, 
in railing the temperature of bodies, is not proportioned 
to their adtion upon the fenfe of vifion. The influence 
of the red light from a fire of coals, and of the white 
light from flame, upon the thermometer, were compared 
together: when the intenfityof their aflion upon the or¬ 
gan of fight was apparently the fame, it was found that 
the effedt of the red light upon the inflrument was in¬ 
comparably greater than that of the white light. When 
bodies are heated to incandefcence, they emit light, in 
which the white or compound fpecies prevails; but, as 
their temperature diminifhes, the light gradually be¬ 
comes of the red fpecies. It would appear, from this 
obletvation, that the light emitted from bodies, while 
they pais from the ftate of incandefcence to the com¬ 
mon temperature of the atmofphere, has its power of 
exciting vifion diminlfhed in a greater ratio than its 
power of influencing temperature. When the light emit¬ 
ted from a body ceafes to be vifible, we are not to con¬ 
clude that the body ceafes to radiate light. The fact 
proves no more, but that the light radiated is unable to 
excite our fenfe of vifion. This light emitted from bo¬ 
dies, without affedling fenfibly the organ of fight, is call¬ 
ed, by Dr. Hutton, obfeure or invilibJe light. 
We have feeiy that the light emitted from bodies, as 
they pafs from the Hate of incandefcence to the common 
temperature of the atmofphere, has its power of exciting 
vifion diminiflied in a greater ratio than its power of 
influencing temperature. It may therefore happen, that 
our obfeure or invifible light fhali flill retain the power of 
fenfibly influencing temperature; and this fufpicion will 
be confirmed, if we find that a fubflance exhibiting the 
dillindlive properties of light is radiated from a body of 
elevated temperature, but not luminous; and if we find 
that this radiated fubflance is capable of railing the ther¬ 
mometer. But this is precifcly what happens in the ex¬ 
periments made by Sauffure and Pidlet. Two concave 
mirrors are placed oppofite to one another, at the dis¬ 
tance of many feet, the bulb of a thermometer is in the 
focus of the one, and a glafs matrafs, filled with boiling 
water, or an iron bullet heated fo as to be juft not vifi¬ 
ble in the dark, is placed in the focus of the other. A 
fubflance is radiated.from the matrafs or iron bullet; it 
palfes with incalculable velocity through the air; is re¬ 
flected from the mirrors, and concentrated according to 
the laws of light, and then influences the thermometer 
placed in the focus, according to the degree of its con¬ 
centration. 
It is fuppofed by Sauffure and Pidlet, that the fubflance 
radiated in thefe experiments, is mot any fpecies of light, 
but what Lambert and Sauffure calls chaleur obfeure ; that 
is, caloric unaccompanied with light. But the proper¬ 
ties of this radiation, its velocity and reflexibiiity, iden¬ 
tify it with light, while they are inconfiflent with the flow 
dilfulion of caloric, and with the known dilpofition of 
caloric to be arrefted by the bodies through which it 
paffes, or with which it comes into contadl. The above 
experiment, made by M. Pictet, ferves to confirm this 
reafoning. He blackened the bulb of bis thermometer, 
and found that it was then more fpeedily influenced by 
the radiation than before, and that it role to a greater 
height. But it is notorious, that black fuvfaces abforb 
light more powerfully than white lurfaces do, and have 
their temperature more confiderably a fie died by it. It 
may be urged, in objedlion to this conchuion, that if we 
fuppofe 
