Explosives may be either mere mixtures, such as gunpowder, or chemical compounds, such as nitroglycerin. They may all be made to explode under sufficient heat; for example, dynamite or nitroglycerin at 180°C., blasting powder at 270°C. and rifle-powder at 270°C. The oldest of the explosives is gunpowder, which is a mixture of potassium nitrate, sulphur and charcoal, in some such proportion as 75 per cent., 10 per cent. and 15 per cent. respectively. The principle of explosives is to bring oxygen into very close contact with combustible substances. Thus even finely powdered wood-dust in the air may be an explosive. When the combustion is simultaneous, it is called detonation. The powerful explosive called guncotton, discovered in 1845, is made by treating cellulose with nitric acid. Guncotton is used in smokeless powder. Nitroglycerin, discovered in 1847, is made by treating glycerin with a mixture of nitric with sulphuric acid. Dynamite is made by combining nitroglycerin with an absorbent substance that prevents it from decomposing, as it would otherwise do. Explosives, chiefly for military use, are an important item of manufacture in the United States and the total value of these products increased from $10,993,131 in 1890 to $40,139,661 in 1909.