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EXPLOSIVES 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.  Expositions. National and international exhibitions or expositions of manufactures, arts, commerce, agriculture and mining are a feature of the present and the recent century. The first of any importance was held at Paris in 1798, and for half a century these displays were held every three years. In this, as in other things, Paris set the fashion. Between 1820 and 1850 exhibitions were held at Vienna, Berlin, Brussels, St. Petersburg, Stockholm, Moscow, Lisbon, Madrid, Dublin, Manchester, Leeds, Birmingham, New York, Philadelphia and many other cities. The first really national exposition in England was held in 1851 at the Crystal Palace, London; and the first of great importance in the United States was held in New York in 1853. The idea was also adopted in the east, and a large exposition was held at Constantinople in 1863. These great industrial shows had now become quite common events, and we need only mention as instances in the next few years the magnificent Paris Exposition of 1867 and the one at Vienna in 1873. The Philadelphia Centennial Exposition of 1876 was held in commemoration of the hundredth anniversary of American independence. Other recent expositions in this country have been those at Louisville in 1882 and at New Orleans in 1884. The Paris Exposition of 1878, which surpassed all others before it, was overshadowed by the Universal Exhibition of Paris in 1889. The (which see), wonderful as it was, was rivaled by the Machinery

Palace, which had a span of 377 feet, but was without pillars or like support.

The World's Columbian Exposition, in Chicago in 1893, far exceeded all its predecessors. The opening of this century was marked by two industrial exhibitions of some magnitude and attractiveness, one at Glasgow, Scotland, and the Pan-American Exposition at Buffalo, N. Y. The design of the latter was to illustrate the progress of the age in science, arts and industries—especially those of the New World. The exposition also had for its object to promote trade among all American countries and to present a great object-lesson showing the industrial development of the continent. Part of its motive-power was obtained from the neighboring Falls of Niagara. The St. Louis or Louisiana-Purchase Exposition of 1903 was held to commemorate the centenary of Jefferson's purchase of the Louisiana territory; the Portland, Oregon, Exposition of 1905 memorialized Lewis and Clarke's successful conclusion of their exploration of the new acquisition; and the Jamestown, Virginia, Exposition celebrated the tercentenary of the founding of the first English colony in the present United States. Seattle, Washington, held an Alaskan Exposition in 1909. The Pan-American Exposition of the resources of the Americas at Buffalo in 1901, which was otherwise a brilliant success, was saddened by the assassination of President McKinley.  Express Companies. This is an American system organized for the speedy transmission of parcels or merchandise of any kind and their safe delivery in good condition. It began in the trip made from Boston to New York by William Frederick Harnden (1813-45), the first “express-package carrier,” on March 4, 1839. The plan recommending itself to business men, competing companies sprang up rapidly, and express lines were established in all directions. Adams & Company's California express was started in 1849; Wells, Fargo & Company's in 1852; the American-European Company in 1855. As railroads extended, the early “pony express” disappeared, and individual companies made contracts with the railroad companies, their business over these routes being held to be entitled to the protection of the courts against any efforts to dispossess them. Many of the rival companies united, and now are joint-stock institutions, employing capital amounting to over $50,000,000. A feature of the system is the “collect on delivery” business—goods to strangers being marked C. O. D., with the amount to be collected, the express company making the collecting and forwarding “returns” to the shipper. Money-orders also are issued, payable at any of the numerous offices throughout the United States. They are received at banks from depositors, and