648 DE. E. EEANICLAND ON THE INFLUENCE OF ATMOSPHEEIC PEESSUEE 
These numbers approximate to those calculated in accordance with the law already 
given for pressures below that of the atmosphere, thus confirming that law for pressures 
up to two atmospheres, as is seen from the following comparison, in which the experi- 
mental numbers are reduced to the standard of 100 at atmospheric pressure. 
Pressure. 
Illiuninating power. 
Observed. 
Calculated. 
1 Atmosphere 
100 
100 
2 Atmospheres first . . 
263-7 
253 
2 Atmospheres second 
261-3 
253 
Further determinations, in which the illuminating power at three- and four-atmo- 
spheres pressure was compared, yielded results differing widely from this law, and indi- 
cating a much more rapid increase of light ; but as the liability to errors of observation 
increases greatly at these higher pressures, I place very little confidence in the numbers 
obtained, which I will nevertheless here briefly state in the same form as the last Table. 
In these experiments the lamp was fed with a mixture of five parts of vinic alcohol and 
one part of amylic alcohol. The lamp when fed with this mixture had no appreciable 
illuminating effect under ordinary atmospheric pressure. 
Pressure. 
Illiuninating power. 
Observed. 
Calculated. 
3 Atmospheres 
406 
406 
4 Atmospheres 
959 
559 
In endeavouring to trace the causes of this variation of luminosity, it will be con- 
venient first to consider the general conditions upon which the light of flames depends. 
The luminosity of the flames generally used for artificial light emanates from two 
sources ; viz., first, from the ignition of minute particles of carbon floating in the 
shell of flame; and secondly, from the incandescence of gaseous matters. The latter 
source of illumination probably does not usually furnish more than one per cent, of the 
total amount of light ; consequently nearly the whole of the light given out by flames 
under ordinary circumstances is due, as Davy first pointed out*, to the ignition of solid 
carbonaceous matter. The light emitted by incandescent gaseous particles becomes, 
however, much more prominent at very low pressures ; and as this light is not materially 
influenced by pressure, it causes the deviation from the law of diminution of light, seen 
at the lower extremities of the lines A C and A D in Plate XIX. diagram No. 1. In order 
to gain a clear conception of the mechanism of a candle- or gas-flame, we must picture 
to ourselves first a core of gas or vapour containing hydrocarbons, and secondly a shell 
* Philosopliical Transactions, 1817, p. 64. 
