Gunpowder was one of the first explosives. The invention of gunpowder brought about great changes in the world. It brought about the end of the days of the robber barons and the beginning of strong nations. Ever since gunpowder was first used in war, people have worked to find better and better explosives to use in fighting. The hydrogen bomb of today explodes with more force than the inventor of gunpowder could imagine.
   But no one should think that explosives are only for war. Mining coal, building tunnels, and clearing land are a few kinds of work in which explosives are important. A series of small explosions, moreover, makes a gasoline engine run. The explosive used is a mixture of air and gasoline.
   Often an explosion is caused by very rapid burning. Gases formed by the burning expand outward with great force. But sometimes in an explosion a compound is merely jarred apart. Nitrogen iodide, for instance, can be made to explode by the touch of a feather.
   Nitrogen iodide has nitrogen in it. So do many other explosives. Nitrogen is not a good joiner. When it does join other materials, it may be rather "restless," and may break away and cause an explosion.
   Dynamite, TNT, and nitroglycerin are three explosives most people have heard about. More powerful explosives have been developed, but these three are still the explosives most often used for peaceful purposes.