350 APPENDIX X
combustion of any substance. This time element varies with the
temperature of the combining bodies according to an exponential
law. High-velocity combustion may be extremely desirable in
artillery practice, or in a very high-speed gas engine, but it would
be of doubtful utility in a boiler setting or an industrial furnace.
The velocity of the reaction is dependent upon mixing as well
as temperature; that is, the same amount of gas burned per
unit of time will give a short flame when jetted through numerous
small holes and a long flame when burned in one jet.
The facility with which gaseous fuels may be mixed with their
air supply is one of their inherent advantages over liquid and
solid fuels. The liquid fuels fall into an intermediate class, while
liquids which have a low evaporative temperature are inter-
mediate between those of high boiling point and the gases.
The investigations of Mallard and Le Chatelier determined the
heat capacity and specific heats of several gases, the heat capacity
of any substance being the amount of heat which will be absorbed
by or given up by that substance when heating or cooling between
the absolute temperatures of TO and TI. The specific heat of
this substance at any given temperature may be derived from its
heat capacity, or the amount of heat that will be absorbed in
increasing its temperature from TQ to TI. In all heating and
cooling operations, the heat capacities of the substances involved
are much more important than their specific heats, as the function
of the operation is heat interchange. The heat capacities fix the
amount of heat that may be given up by one substance as well
as the amount of heat absorbed by the other substance, between
the temperature limits involved.
Therefore these curves, in addition to showing approximately
the temperature that may be realized from the combustion of the
particular combustible, give the amount of heat that will be
available or given off by the products of combustion in cooling
from a high temperature to a lower one.
The utilization of this heat will depend upon:
(a) The ruling temperature of the operation, that is, the
upper and lower temperatures required;
(6) The design of the heating chamber;
(c) The heat capacity of the substance to which the heat is