Other illustrations are offered by gunpowder and nitro-glycerine or dynamite. These are both very essential agents in civilization. Our greatest labors in mining and engineering would be impossible without nitro-glycerine. The two materials supplement each other. Powder alone can be used as a projectile agent, and is extremely serviceable in blasting when only a moderate force is required. In works of magnitude nitro-glycerine is "vastly superior to gunpowder," so much so as to be in many cases a necessity if these labors are to be prosecuted.

This superiority is due to its chemical construction. The explosive powers of both materials arise from the sudden production of gas. "Nitroglycerine yields fully nine hundred times its volume of gas, while with gunpowder the volume is only about three hundred times that of the solid grains."* A second consideration of more moment is that the explosion in the case of glycerine is instantaneous, the decomposition extending at once to the whole mass; in gunpowder it is simply rapid. In both materials the reaction is due to the weak affinity of nitrogen, and the explosive force to the nitrogen, liberated as gas, and to the carbonic acid gas, the result of the reaction. "But while in the gunpowder, the carbon and oxygen atoms are in different molecules, although lying side by side in the same grain; in the nitro-glycerine they are in different parts of the same molecule." + The ease and safety in the use of gunpowder are found in

*The New Chemistry, p. 224.

+ Ibid, p. 220.