371 



In tracing the cause of this variation of light under different 

 atmospheric pressures, the author calls attention to the conditions 

 upon which the light of ordinary flames depends. He shows that it 

 is derived almost exclusively from the separation of carbon particles 

 within the flame, and that it is increased by the augmentation of the 

 amount of carbon thus precipitated, and by an increased temperature 

 in the flame ; whilst it is diminished by the separation of less carbon 

 and by a reduction of temperature. The temperature of flame is. 

 not materially altered by the rarefaction of the 'supporting medium ; 

 and hence the loss of light cannot arise from a reduction of tempera- 

 ture. On the other hand, the separation of carbon particles is 

 greatly augmented by increased pressure ; thus candles evolve much 

 smoke when burnt under a pressure of two atmospheres ; whilst even 

 a small alcohol flame, which burns with a pure blue light at ordinary 

 pressures, becomes highly luminous in air four times compressed. 

 Flames which smoke at ordinary pressures become smokeless in 

 rarefied air, and undergo more complete combustion. Whilst, there- 

 fore, the light of flames is due to the separation of carbon particles, 

 the latter owe their momentary existence to the absence of sufficient 

 oxygen for their combustion ; consequently any influence which 

 causes the more rapid interpenetration of the flame gases and exterior 

 air, must reduce the amount of precipitated carbon, and consequently 

 also the luminosity of the flame. Rarefaction exercises precisely 

 such an influence by increasing, as it is well known to do, the 

 mobility of the gaseous particles, and thus causing the access of a 

 larger amount of oxygen to the region of the flame where precipitated 

 carbon produces luminosity. 



An analysis of the gases evolved from a candle flame, burning 

 under a pressure of only eight mercurial inches, proved that there 

 was perfect combustion even at this low pressure. 



In conclusion, the influence of atmospheric pressure upon the 

 phenomena of combustion may be thus summed up. 



1 . The rate of burning of candles and other similar combustibles, 

 whose flames depend upon the volatilization and ignition of com- 

 bustible matter in contact with atmospheric air, is not perceptibly 

 affected by the pressure of the supporting medium. 



2. The rate of burning of self-supporting combustibles, like time- 

 fuses, depends upon the rapidity of fusion of the combustible com- 



