1817.] Observations on the Flame of a Candle. 341 
Thus the supposed opacity of flame is nothing more than this: 
the rays emitted from a weak light cease to affect the eye while that 
organ is affected by a stronger one. 
But I shall not content myself with proving that flame is not an 
opaque body, but adduce the following experiment to show that it 
is a remarkably transparent one :— 
Place a lighted spirit lamp about three feet from a sheet of white 
paper stuck against a wall, and about nine feet from a lighted 
candle, so that the lamp may be between the wall and the candle. 
Close by the side of the flame of the lamp put a small piece of thin 
and clear glass. ‘Then look at the sheet of paper, and two very 
faint shadows will be seen upon it: the one will represent the flame 
of the spirit lamp, and the other the piece of glass; but the shadow 
of the glass will be the deepest : consequently the flame intercepts 
less light ; or, in other words, is more transparent than glass. 
The opacity of flame will not now, I presume, be urged against 
the explanation which I have given of one of the obscuring effects 
of the wick. I proceed now to mention the other, which depends 
on its conducting power. A Jong wick conducts downwards a con- 
siderable portion of the heat of the flame, which is expended in 
volatilizing more tallow (hence the increased consumption of that 
substance) ; and by thus diminishing the temperature of the flame, 
the particles of charcoal are not ignited to such an intense degree as 
they otherwise would be, and consequently they do not emit so 
much light. Hence whatever conducts off much caloric from the 
flame diminishes)its light. I have been able to reduce the tempe- 
rature of the flame of a candle so considerably by placing it in an 
impure atmosphere, at the same time that a metallic sphere of the 
size of a musket-ball was suspended so as to dip deep into the flame, 
that it burned with a feeble blue light, like that of spirits of wine. 
A similar effect takes place at the bottom of the flame of a candle : 
the small quantity of gas and vapour emitted from this part of the 
wick burns here with a feeble blue flame, in consequence of its 
proximity to the wick replete with tallow passing to the gaseous 
state, by which its temperature is kept down. In the lighted stream 
of coal gas the same cooling effect is produced near the bottom by 
the constant current of cold gas. In all these cases the heat of these - 
parts is not sufficient to decompose any portion of the gas inclosed 
within the outer flame; consequently no charcoal is separated and 
ignited; and therefore (conformably to the principle discovered by 
Sit H. Davy) the emission of light is very trifling. 
The preceding observations have led me to attempt an improve- 
ment in the construction of lamps for burning oil. Should this 
attempt succeed to my expectations, I may possibly at a future time 
lay the result before the public. 
With much esteem I remain, 
Dear Sir, yours very truly, 
Tower, Jan. 28, 1817, R. Porrerr, jun. 
