234 COMBUSTION OF GUN-COTTON AND GUNPOWDER. 
In comparing the results of the combustion of gun-cotton in a vacuum with 
those produced by the explosion of an iron cylinder filled with it, it will be per- 
ecived that the change from one series of products to the other involves an 
increase in the volume of the evolved gases—an effect due chiefly, but not 
wholly, to the heat produced by the chemical reaction between the nitric oxide 
generated by the first act of combustion and the carburetted hydrogen present. 
When gun-cotton, therefore, burns in a sufficiently strong and well-filled ves- 
sel, it is resolved into gaseous products which immediately react on each other 
with an increase in the temperature and tension of their mass, and on the sud- 
denness of this reaction is probably due some part of the great percussive force 
developed by the explosion of gun-cotton in strong vessels. 
I believe that no determination has been made of the amount of heat evolved 
by the explosion of gun-cotton. 
The sums of the heat, and of the mechanical effects representing heat, pro- 
duced by equal weights of gun-cotton and of gunpowder would not be very 
different if assumed to be proportional to the amounts of oxygen concerned in 
the chemical reactions in each case, but the greater volume of the gases evolved 
from gun-cotton makes their actual temperature less and their mechanical effect 
greater.* 
The much greater heating effect, however, which gunpowder exerts upon the 
gun from which it is fired is to be attributed not only to the higher temperature 
and greater density of the products of explosion, but to the circumstance that 
in the case of gunpowder sulphide of potassium is deposited on the walls of the 
gun, probably from a state of vapor, imparting to the cold metal both its free 
heat and its heat of condensation, the action being analogous to that of steam, 
which, in condensing on a cold body, heats it much more rapidly than a cur- 
rent of a non-condensible gas of the same temperature could have done. 
B. F. CRAIG. 
*Tt may be here remarked that the comparative mechanical energy developed in fire-arms 
by gunpowder and by gun-cotton is to be estimated not by the amount of motion imparted 
to the projectile, but by that imparted to the gun. 
These two are different things, and the latter must always be greater than the former by 
an amount equal to the vis viva with which the products of decomposition of the projectin 
agent are expelled from the gun, and this vis viva must, of course, vary with the weight of 
the explosive material. 
This consideration makes it evident why, when a lesser weight of gun-cotton is substi- 
tuted for a greater weight of gunpowder, the recoil of the gun is less, while the velocity of 
the shot may be unchanged. B. F.C. 
