160 On the Production of Light by Chemical Action. 
chemical conditions that determine these singular differences? 
How is it that by changing the circumstances of combustion we 
ean vary the nature of the light? We turn aside the flame of a 
candle by means of a blowpipe, — a neat blue cone appears. 
Why does it shine with a blue light? 
Such enquiries might be multiplied without end: but a little 
consideration shows, that their various answers depend on the 
determination of a much more general problem; viz.—Can any 
connection be traced between the chemical conditions under which 
a body burns, and the nature of the licht it emits? It is to the 
discussion of that problem that this memoir is devote 
Sir vy has already furnished us with two important cir- 
cumstances in relation to the nature of flame; Ist, all common 
flames are incandescent shells, the interior of which is dark ; 
2nd, a paar quantity of light emitted depends on the tem- 
porary disengagement of solid particles. 
It is ae by a very general examination of the lght pang 
from various solids, vapors, and gases, when burning, that 
can expect to obtain data for a true theory of combustion. This 
is what I shall endeavor to furnish on the present occasion. 
As was foreseen by all the older chemists, the true theory of 
combustion, whatever it may prove to be, must necessarily be 
one of the He ake theories of chemistry. It must include 
the nature of all chemical changes whatsoever. The subject is 
therefore not alone interesting in a popular sense, but of great 
importance in its scientific connexions. 
I. Prismatic analysis of the flames of various tt nl and 
gases, proving that they yield ali the colors of the spectrum. 
I commenced this investigation of the nature of ‘feme, and of 
combustion generally, by an optical examination of various bodies 
in the act of burning. Some authors have asseried that certain 
flames yield monochromatic lights. It is necessary to verify this 
assertion if true, or set it aside if false. 
The instrumental arrangement which I have employed is as 
follows: the rays of the flame of which ake examination is to 
be made, pass through a horizontal slit ;'; inch wide and one 
inch long i in a metallic screen, and are se at a pea: of six 
or eight feet on a flint glass prism, the axis of which is paralle 
to the slit. After passing the prism they enter a small telescope, 
which has a divided micrometer, and also parallel wires in its eye- 
piece. ‘Through this telescope the resulting spectrum is viewed. 
If it be the flame of a lamp of any kind ‘that is to be examin- 
ed, by using a moveable stand, we are able to raise or lower it, 
and thus analyze different horizontal elements, in its lower, its 
middle, or its upper part at pleasure. If, i instead of a horigonell 
we wish to examine a vertical element t of the flame, the . 
