16 
MINUTES OF PEQCEEDINGS OF 
latter is not affected by pressure; and as the temperature of the flame is 
also proved to remain practically constant, effecting the evaporation of the 
same amount of combustible matter under all pressures, it follows that the 
rate of consumption of a candle must, be nearly or quite independent of the 
pressure of the surrounding medium. In the deflagration of time-fuzes, the 
conditions are obviously very different. Here the combustible matter never 
comes into contact with atmospheric oxygen until it has been ejected from 
the fuze-case. CJnlike 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 fuze composition, the whole would burn 
almost instantaneously. Under ordinary circumstances, however, the fuze 
burns only at a disk 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 temperature at which chemical com¬ 
bination 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 unburnt layer must depend, in great measure, 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 disk 
of composition will also be diminished. Hence the slower rate of deflagration 
in rarefied air. 
