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LEFT EXPLOSIVES 78 EXPRESS but burns with a smoky flame. It can, however, be detonated by means of ful- minate of mercury. It is safe to handle and remarkably stable, and while slight- ly less powerful than picric acid it has the advantage that it does not attack metals or form unstable compounds with them. Picric acid, or trinitrophenol, occurs as bright yellow crystals, but is usually fused when used as an explosive (v. Lyddite). Picric acid does not explode easily by direct percussion, but it can be readily detonated by mercury ful- minate. Nitroglycerin is prepared by nitrating glycerine in the presence of sulphuric acid. It is a heavy, oily liquid, having a specific gravity of 1.60. Owing to the Smokeless powders are mixtures of nitrocellulose or nitrocellulose nitro- glycerin, with other ingredients added as stabilizers, "deadeners," or "coolers." The explosive is dissolved (usually in acetone or a mixture of ether and al- cohol), the mixture is rolled into sheets, the solvent evaporated, and the powders finally dried at a temperature of about 40° C. Cordite belongs to this class, and consists of a mixture of nitroglycerin and guncotton, with a small amount of vaseline, gelatinized by means of ace- tone, and dried. Percussion caps are filled with explos- ive compositions, of which the chief con- stituent is mercury fulminate (Hg (CN0)2). When used alone, however, this substance is too rapid in its action WAsrt ACIO E6G /wash /wash FlLTEl^ FILTEI? EXPLOSIVES — DIAGRAM OP MANUFACTURE OF TRINITROTOLUENE (T.N.T.) great danger of premature explosion during transportation, nitroglycerin is no longer used as such. It forms, how- ever, an important constituent of d/yyui- mite, which consists of nitroglycerin, absorbed on a base, which may be inert, combustible or explosive. The absorb- ents chiefly used are kieselguhr, mag- nesia, charcoal, and wood pulp. Ex- plosive bases are usually nitrates of potassium, sodium, or ammonium or or- ganic nitro derivatives. Guncotton, or trinitrocellulose, is manufactured by nitrating cotton with a mixture of nitric and sulphuric acids. It is a grayish-white solid, odorless and tasteless, and, when properly prepared, is a safe and permanent explosive. and it is, therefore, mixed with such materials as potassium chlorate, sul- phur, antimony sulphide, and powdered glass. Other explosive compounds have been prepared by nitrating starch, sugar, dex- trin, gelatin, resin, and even coal, but these are of minor importance. EXPRESS, in the United States, a system organized for the speedy trans- mission of parcels or merchandise of any kind, and their safe delivery in good con- dition. It originated in the trip made from Boston to New York by William Frederick Harnden (1813-1845), the first "express-package carrier," March 4, 1839. The project recommending itself to busi-