636 DE. E. EEAJ^^KLAifD ON THE INELIJENCE OE ATMOSPHEEIC PEESSUEE 
in which the candles were burnt, must have occasioned a comparatively high temperature 
in the atmosphere of the chamber, necessarly causing the candles to gutter. Further, 
the very imperfect combustion which a candle undergoes at this high pressure would 
have a tendency to increase the size of that portion of the wick situated within the flame, 
and which constitutes the surface from which the evaporation of the combustible proceeds. 
Both these circumstances would, I conceive, practically tend greatly to shorten the time 
during which a candle would burn, which was precisely the circumstance that alone 
attracted M. Teigee’s attention, no quantitative determinations of the weight of com- 
bustible matter actually consumed having been made. 
In the deflagration of time-fuses, the conditions are obviously very different. Here the 
combustible matter never comes into contact with atmospheric oxygen until it has left 
the fuse-case; unlike the candle, the composition contains within itself the oxygen 
necessary for combustion, and a certain degree of heat only is necessary to bring about 
chemical combination. If this heat were applied simultaneously to every part of the 
fuse-composition, the whole would burn almost instantaneously. This sometimes 
approximately occurs, when, by the expansion of the wooden case into which the com- 
position is rammed, a slight space is formed between the case and its contents, thus 
allowing the deflagration to propagate itself between the case and the composition. 
Under such circumstances the fuse burns with explosive rapidity; and probably the occa- 
sional bursting of shells before, or immediately after leaving the guns, may be due in some 
cases to this cause. Under normal circumstances, however, the fuse burns only at a disc 
perpendicular to its axis ; and the time occupied in its deflagration necessarily depends 
upon the rapidity with which each successive layer of composition is heated to the tem- 
peratm’e at which chemical combination takes place. This heat, necessary to deflagration, 
is evidently derived from the products of the combustion of the immediately preceding 
layer of composition ; and the amount of heat thus communicated to the next unbumt 
layer must depend, in great measm’e, upon the number of particles of these heated 
products which come into contact with that layer. Now, as a large proportion of these 
products are gaseous, it follows that, if the pressure of the surrounding medium be reduced, 
the number of ignited gaseous particles in contact at any one moment with the still 
unignited disc of composition will also be diminished. Hence the slower rate of defla- 
gi'ation in rarefied air. 
II. INELHENCE OE ATMOSPHEEIC PEESSHEE ON THE LIGHT OE COMBHSTION. 
a. Influence of Rarefaction. 
In burning candles upon the summit of Mont Blanc, I was much struck by the compara- 
tively small amount of light which they emitted. The lower and blue portion of the 
flame, which, under ordinary cu'cumstances, scarcely rises to within a quarter of an inch 
of the apex of the wick, now extended to the height of one-eighth of an inch above the 
cotton, thus greatly reducing the size of the luminous portion of the flame. 
On returning to England, I repeated the experiment under cu’cumstances which 
