_Nitro-glycerine._—This is formed by the action of nitric and sulphuric acids on animal compounds such as gelatine or glycerine. Nitro-glycerine is a yellowish, oily, highly unstable explosive liquid with a specific gravity of about 1.6. It will burn quietly when ignited in the open air. It will freeze at 41 degrees F., and will explode at 388 degrees F., or on concussion at a lower temperature. It develops about 1,500 times its volume in gas, which due to the heat of combustion is increased to about 10,000 times its volume. It is a very dangerous explosive to handle, and is unsuitable for use in the liquid form. _Blasting Gelatine._—This is made by soaking guncotton in nitro-glycerine. Gelatine dynamite is a combination of blasting gelatine and an absorbent. Forcite is a gelatine dynamite in which the blasting gelatine, forming 50 per cent of the compound, contains 90 per cent nitro-glycerine and 2 per cent guncotton; and the absorbent, forming the other 50 per cent of the compound, contains 76 per cent of sodium nitrate, 3 per cent sulphur, 20 per cent of wood tar, and 1 per cent of wood pulp. Blasting gelatine is packed in a jelly-like mass in metal lined wooden boxes. It is less sensitive than straight dynamite and is one of the most powerful explosives known. It can be made up to equal 100 per cent dynamite. It is suitable for use in the hardest rocks and for subaqueous work as it is not affected by moisture. It is suitable for use in tunnels as the amount of carbon monoxide, peroxide of nitrogen, hydrogen sulphide and other dangerous gases is comparatively low when fully detonated. Gelatine dynamite[98] is sold as 30 per cent to 70 per cent dynamite, the actual percentage of nitro-glycerine being less than the nominal quantity given. _Dynamite._—The dynamites are made by soaking nitro-glycerine in some absorbent. If the absorbent is some neutral substance such as infusorial earth the combination is known as a true dynamite. The false or active dynamites are those in which the absorbent is also an explosive compound. The false dynamites form the best known contractors’ explosives. Among the materials mixed with the nitro-glycerine are: magnesium carbonate, sulphur, wood meal, wood pulp, wood fiber, wood tar, nut galls, kieselguhr, sawdust, resin, pitch, sugar, charcoal, and guncotton. The strength of dynamites is noted by the per cent of nitro-glycerine and nitro substitutes contained. Dualin and Hercules powder both contain 40 per cent nitro-glycerine. Dualin contains 30 per cent sawdust and 30 per cent potassium nitrate, but the Hercules powder, which is stronger, contains 16 per cent sugar, 3 per cent potassium chlorate, 31 per cent potassium nitrate, and 10 per cent magnesium carbonate.