L I G H T. 
tiplying tliefe ratios, wo get 270 to S, or 54 to 1 nearly, 
for the relative intenfity of the light of each. 
Thefe different gafes are here 1 'uppofed by Dr. Henry 
to be mixtures of l’everal gafes the compofition of which 
is known, and all conlilting of different proportions of 
hydrogen and carbon, with the exception of the carbonic 
oxyd, wftich contains oxygen. Now, letting afide the 
latter gas, which being partly faturated, the reft will be 
in their effect to produce light, as the quantity of carbon 
they contain; not that carbon is more effeflive than hy¬ 
drogen, but becaufe the f'pecific gravity of the gas is in- 
creafed by the carbon. Since, therefore, the olefiant gas. 
contains the moft, and is of the greateft fpecific gravity 
in confequence, thofe gafes which contain the greateft 
quantity of olefiant gas muft be the beft for producing 
3 ight by combuftion. Now, it may be leen in the table, 
that the gafes obtained from lamp-oil and wax are the 
next to the olefiant gas in their effeft of giving light; and 
hence we may conclude that thofe gafes contain a large 
proportion of olefiant gas, and of courfe feem well fitted 
for producing artificial light. If we may judge by ana¬ 
logy, we may expeft that the fpermaceti fat will ltand as 
high as the oil; and the beft tallow may, perhaps, be the 
next in order. During the burning .of thefe fubfiances, 
the vapour which, when kindled, conftitutes the flame, 
may confift of a great proportion of the olefiant gas. In 
the burning of all fatty fubfiances, however, there is a 
great redundancy of carbon, which flies off with the ga- 
feous products in the form of fmoke, and which is burnt 
in the Argand lamp. The above faiFts will furnifh an 
elegant and Ample method of appreciating the relative 
value of the different combuftibie bodies to produce 
light. 
The light which is furnifhed by combuftion, and com¬ 
monly called artificial light, is confidered by moft philofo- 
phers as being a component part of the bodies employed 
in the combuftion. Even in the Ample procefs of heating 
a body red-hot, it is faid that the body at that tempera¬ 
ture begins to give out light. It is rather curious, that 
all bodies fhouid give out light at the fame temperature, 
which is. faid to be about 8oo° Fait. It would appear, 
from a fa-T given by the late Mr. Wedgewoocl, that the 
emanating medium is heat or light, according to the den- 
fity of the body from which it is emanated. The heated 
air is fo hot, as to make a thin flip of gold appear red-hot, 
although the aerial medium did not become luminous. 
Terreftrial light, as it appears to poflefs molt of the pro¬ 
perties of folar light, like it can be tranlmitted and re- 
irafted by tranfparent bodies ; and it ftriftly refembles it 
in being reflected by the fame bodies. It is faid, however, 
to contain a different proportion of the coloured rays 
from that of folar light, being defeftive in the blue and 
redundant in the red rays. 
Dr. Herfchel has made a number of experiments upon 
terreftrial heat, in order to compare it with the heat fur- 
nilhed by the fun; but he feems to have taken it for 
granted, that the folar light and the artificial do not dif¬ 
fer, although his experiments prove that terreftrial heat 
and folar heat differ effentially. Artificial heat does not 
pafs through glafs ; while the folar calorific rays pafs eafily 
through the iubftance of a prifm, and afterwards through 
a convex lens. It has been found that artificial light has 
fome chemical properties. The abbe Teflier found that 
the green colour of vegetables is produced by the light of 
a lamp ; which faft has been confirmed by Decandolle. 
The purpofes for which mankind employ fires, or com¬ 
buftion in general, are either for the ufe of the heat, or 
for the ufe of the light. The heat is fubfervient to the 
numerous and important purpofes of cooking victuals, of 
warming apartments, and thus rendering inhabitable l'uch 
climates as otherwife the human 1’pecies could not live in ; 
of giving exiftence to all metallurgic operations, .to the 
making of glafs, of lime for building, &c. &c. The light 
is fubfervient to purpofes equally important. It enables 
human beings to follow their operations, during the ab- 
fence of the day-light, nearly if not full as well as in the 
Vol. XII, No. 862. 
631 
day-time. The flame of a Angle candle animates a fa¬ 
mily ; every one follows his occupations, and no dread is 
felt of the darknefs of night. Were it not for artificial 
light, how great a portion of the advantages of induftry, 
and of real comfort, would the human fpecies be de¬ 
prived of! 
When heat is wanted, then rough folid combuftibles are 
ufed, which give it in abundance, and at a cheap rate ; 
but, when light is wanted, then the pureft and molt 
uniform combuftibles mult be ufed, otherwife an inade¬ 
quate effect is produced, and a coniiderable quantity of 
materials is expended. In fome uncivilized countries, (len¬ 
der faggots of fome kind of refinous wood are tried by 
way ot candles. When lighted atone end, they burn gra¬ 
dually, and afford a good deal of light; but it is unlteady, 
and encumbered with a good deal of fmoke. Befides, 
thefe faggots are readily burnt out, and muft be quickly- 
replaced by new ones. At prefent, in all civilized coun¬ 
tries, the principal combuftibles that are ufed for the pro¬ 
duction oPa bright and luminous flame, are wax, the fat 
of animals, under the general name of tallow, and oil, ei¬ 
ther of fith or of vegetables. The extenfive confumptiort 
of thefe materials, and the fuccefllve increase of their 
price, have obliged the induftrious to devife the beft means 
of producing the greateft effect with the leaft poilible 
quantity of materials; as to which, fee the article Lamp, 
p. 115, 16. 
Of the three principal materials for producing artificial 
light, viz. wax, tallow, and oil, the firft and fccond are 
moftly ufed within-doors in this country ; but fill)-oil, 
the combuftion of which is attended with an unpleafant 
fmell, is moftly ufed for (treet-lamps and other out-of- 
doors purpofes ; excepting indeed when Argand’s lamps 
are ufed, for in thefe the oil gives no bad fmell. Oil of 
olives burns without any offenfive fmell; therefore much 
ufe is made of it for lamps in private houfes in thofe 
countries where it may be had at a cheap rate, as in Italy, 
the Couth of Franee, &c. 
Befides the above, a new material has of late been at¬ 
tempted to be introduced in this country, for the purpofe 
of lighting houfes, ftreets, manufactories, &c. this material 
is the inflammable gas of coals, (properly called carbu- 
retted hydrogen gas,) of which we have juft fpoken in 
our companion of gafes. Every body muft know', that, 
when coals are burning in a common-fire-place, a flame 
more or lefs luminous (according-as it is more or lefs en¬ 
cumbered with incombultible Imoke and vapour) iffues 
from them ; and they frequently emit fome very beauti¬ 
ful ftreams of a flame remarkably bright. All this, as wo 
have already mentioned, arifes from the gafes which are 
extricated from the coals by the heat. It was natural to 
imagine that l'uch gas might be received in proper refer- 
voirs, and might afterwards be forced out of fmall aper¬ 
tures, which, being lighted, might ferve, as the flames of 
candles, to illuminate a room or other place. The trial 
was eafily made, and it was attended with the defired ef- 
feit. The principle of the apparatus and of the opera¬ 
tion is as-follqws : The coal is placed in large iron vef- 
fels, called retorts, to the apertures of which iron pipes 
are adapted, which terminate in a veflel, or veffels, called 
gazometers, gas-holders, or refervoirs, which are inverted 
in water. The retorts thus charged ate placed upon the 
fire, the action of which extricates the gas from the coals 
that are within the retorts, together with an aqueous va¬ 
pour, a thickilh fluid, or tar, &c._ Thefe products are 
conveyed through the above-mentioned pipes under the 
gazometers, where the gas is wafted, and remains ready 
tor ufe. There are then other fmalier pipes from the ga- 
zometer, which branch out into fmalier ramifications, 
until they terminate into the places where the lights are 
wanted. The extremities of the pipes have fmall aper¬ 
tures, out of which the gas iffues; and, the ftreams being 
lighted at thofe apertures, will burn with a clear and con- 
ftant flame as long as the fupply of gas continues: 
Mr. Murdoch was probably the firft perfon who put in 
practice G^s-lights, on an extenfive fcale; he com- 
8 L meacel 
/ 
