the degree of heat neither too great nor too little required to inflame it, but its want of full power is connected with the same cause. Combustion, extending from grain to grain, is not instantaneous, and the first grains of a large charge begin to do their work before the entire charge is inflamed. This fact greatly diminishes the power of gunpowder as an explosive, while making it only the more efficient as a projectile force. The gun would be burst by instant explosion, while a continuous explosion follows up and accelerates the ball. Nitro-glycerine is not exploded by heat but by concussion. "The flame of an ordinary match can be extinguished in it."* Thus concussion reaches all parts instantaneously, or proximately so, and the molecules awake their slumbering energies conjointly.

The many organic compounds of carbon are peculiarly unstable, but carbon as coal is so inert that it can be treasured anywhere in the earth for any period of time. There is to this assertion the one exception on which the value of the coal deposits depends. Coal, at an easily attained temperature, burns, maintains its own combustion, and yields heat and so kinetic energy in manageable ways and amounts. Iron, by whose instrumentality this energy is chiefly turned into working power, oxidizes, on the other hand. slowly at a low temperature, while at a high temperature oxidation ceases. Heat checks the combustion of the furnace, while it promotes that of the coal in the furnace. The heated steam in the boiler acts on it, notwithstanding

* Ibid, p. 216