refrangible ; and has its colour restored, or 
is again oxygenized, in the least refrangible 
rays. 
Certain bodies have the property of ab- 
sorbing the rays of light in their totality, of 
retaining them for some time, and of again 
evolving them unchanged, and unaccom- 
panied by sensible heat. Thus, in an expe- 
riment of Du Fay, a diamond exposed to 
the sun, and immediately covered with 
black wax, shone in the dark, on removing 
the wax, at the expiration of several 
months. Bodies possessing this property, are 
called solar phosphori ; such are Canton’s, 
Baldwin’s, Homberg’s, and the Bologman 
phosphori. To the same class belong 
several natural bodies which retain light, 
and give it out unchanged. Thus, snow is 
a natural solar phosphorus. So also is, 
occasionally, the sea when agitated ; putrid 
fish have a similar property ; and the glow- 
worm belongs to the same class. These 
phenomena are independent ot every thing 
like combustion; for artificial phosiihori, 
after exposure to the sun’s rays, shine in the 
dark when placed in the vacuum of an air- 
pnmp, or under water, &c. where no air is 
present to effect combustion. 
From solar phosphori, the extrication of 
light is facilitated by the application of an 
elevated temperature ; and, after having 
ceased to shine at the ordinary tempera- 
ture, they again emit light when exposed to 
an ihcrease of heat. Several bodies, which 
do not otherwise give out light, evolve it, 
or become phosphorescent when heated. 
Thus, powdered fluate of lime becomes 
luminous when thrown on an iron plate, 
raised to a temperature rather above that 
of boiling water. The yolk of an egg, when 
dried, becomes luminous on being heated ; 
and so also does tallow during liquefaction. 
To exhibit the last mentioned fact, it is 
merely necessary to place a lump of tallow ' 
on a coal, heated below ignition, making 
the experiment in a daik room. Attrition 
also evolves light, in many instances, by 
the part rubbed becoming ignited. Thus, 
rock crystal, and other hard stones, shine 
when rubbed against each other ; and two 
pieces of common bonnet cane, rubbed 
/ strongly against each other in the dark, 
emit a faint light ; most probably from the 
silex they contain : and tw'o pieces of borax 
have the same property much more remark- 
ably. , 
Light is disengaged m various cases ot 
chemical combination, Whenever combus- 
tion is a part of the phenomena, this is well 
known to happen ; but light is evolved also, 
in other instances, where nothing like com- 
bustion goes forwards. Thus, fresh-pre- 
pared pure magnesia, added suddenly to 
highly concentrated sulphuric acid, exhibits 
a red heat. 
Whence comes the light afforded by ig- 
nited bodies ? whether it have been previ- 
ously imbibed by them ? whether the com- 
mencement of ignition be distinctive of the 
same temperature in all bodies ? whether 
the great planetary sources of light be bo- 
dies in a state of combustion, or merely lu- 
minous upon principles very difierent from 
any which our experiments can point out ? 
whether the momentum of the particle* 
of light, or their disposition for chemical 
combination, be the most effectual in the 
changes produced by its agency? these, 
and numerous other interesting questions, 
must be left for future research and investi- 
gation. See Combustion. 
The production of light by inflammation 
is an object of great importance to society 
at large, as well as to the chemist. It ap- 
pears to arise immediately from the strong 
ignition of a body while rapidly decom- 
posing. Most solid bodies in combustion 
are kept, partly from a want ot the access 
of air, and partly from the vicinity of con- 
ducting bodies, at a low degree of ignition, 
But when vapours rapidly escape into the 
air, it may, and does frequently happen, 
that the combustion, instead of being car- 
ried on merely at the surface of the ma^, 
penetrates to a considerable depth within, 
and from this, as well as from the imper- . 
feet conducting power of the surrounding 
air, a white heat, or very strong ignition, is 
produced. The effect of lamps and can- 
dles depends upon these considerations. A 
combustible fluid, most commonly of the 
nature of fat oil, is put in a situation to be 
• absorbed between the filaments of cotton, 
linen, fine wire, or asbestos. The extremity 
of this fibrous substance, called the wick, is 
then considerably heated. The oil evapo- 
rates, and its vapour takes fire. In this si- 
tuation the wick, being enveloped with 
flame, is kept at such a temperature, that 
the oil continually boils, is evaporated, 
burns, and by these means keeps up a con- 
stant flame. Much of the perfection of 
this experiment depends on the nature, 
quantities, and figure of the materiaWmade 
use of. If the wick be too large, it will sup- 
ply a greater quantity of the fluid than can 
be well decomposed. Its evaporation will 
therefore diminish the temperature, and 
