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GENERAL REPORT OF THE JUDGES OF GROUP XVI. Mr. Abel has recently determined the velocity of detonation, trans mitted from disk to disk of dry gun-cotton in contact, to be about seventeen thousand feet per second, while if the disks are saturated with water it is decidedly higher,—say twenty thousand feet. Nitro-glycerine.—This explosive was discovered in 1847 by As- cagne Sobrero, but was first introduced into general use in blasting by Alfred Nobel in 1864. Since that date it has been largely em ployed in Europe and this country. It is the most powerful agent now known; but, uncombined with absorbents, it is justly considered as dangerous in manipulation and as unfit for long storage on account of liability to spontaneous decomposition, or even detonation, unless extreme care has been observed in its manufacture. It is made by slowly introducing pure glycerine into a mixture -of strong nitric and sulphuric acids, especial attention being paid to pre venting a rise of temperature. There are believed to be three nitro glycerines, mono-, di-, and tri-; and the chemical formula of the latter, which should be as exclusively produced as possible, is C 3 H 5 N 3 0 9 . Above 40° Fah. it is an oily liquid having a specific gravity of 1.6, odorless, and of a sweet taste. It is poisonous; and, if placed in con tact with the skin, even in small quantities, before the system has become accustomed to its action, it produces violent headache. Below 40° Fah. it congeals into a white crystalline solid, which is nearly or quite unexplosive, and which may be handled or transported with safety. It may readily be thawed by introducing the can into warm water, which restores its full explosive power. These characteristics are those of tri-nitro-glycerine. Important discrepancies reported in its properties are believed to be caused by various admixtures of the lower nitro-glycerines. Nitro-glycerine, ignited in small quantities by a flame and uncon fined, burns with difficulty like an ordinary oil. At 423 0 Fah. it deflagrates violently. If ignited confined, or if subjected to the ex plosion of fifteen grains of fulminating mercury, it detonates with tremendous force. Fully exploded, it gives off no injurious gases,— an important advantage in tunnel blasting. The accidents which attended the use of nitro-glycerine in its liquid form led Alfred Nobel to experiments with absorbents; and, in 1866-67, he invented and introduced the explosive known as dynamite or giant powder. This consists of seventy-five per cent, of nitro-glycerine and twenty-five per cent, of an inert silicious earth. The best variety of the latter is that known as kieselguhr, found in Hanover, but many others have been employed. Nitro-glycerine in this form possesses great advantages. It is less liable to accidental
