It does not therefore contain sufficient oxygen for the complete combustion of its carbon. It is for this reason that when used for mining purposes a nitrate is generally added to supply this defect (as, for instance, in tonite). It tends also to prevent the evolution of the poisonous gas, carbonic oxide. The success of the various gelatine explosives is due to this fact, viz., that the nitro-glycerine has an excess of oxygen, and the nitro-cotton too little, and thus the two explosives help one another. In practice the gases resulting from the explosion of gun-cotton are-- Carbonic oxide, 28.55; carbonic acid, 19.11; marsh gas (CH_{4}), 11.17; nitric oxide, 8.83; nitrogen, 8.56; water vapour, 21.93 per cent. The late Mr E.O. Brown, of Woolwich Arsenal, discovered that perfectly wet and uninflammable compressed gun-cotton could be easily detonated by the detonation of a priming charge of the dry material in contact with it. This rendered the use of gun-cotton very much safer for use as a military or mining explosive. As a mining explosive, however, gun-cotton is now chiefly used under the form of tonite, which is a mixture of half gun-cotton and half barium nitrate. This material is sometimes spoken of as "nitrated gun-cotton." The weight of gun-cotton required to produce an equal effect either in heavy ordnance or in small arms is to the weight of gunpowder in the proportion of 1 to 3, i.e., an equal weight of gun-cotton would produce three times the effect of gunpowder. Its rapidity of combustion, however, requires to be modified for use in firearms. Hence the lower nitrates are generally used, or such compounds as nitro-lignose, nitrated wood, &c., are used. The initial pressure produced by the explosion of gun-cotton is very large, equal to 18,135 atmospheres, and 8,740 kilogrammes per square centimetre for 1 kilo., the heat liberated being 1,075 calories (water liquid), or 997.7 cals. (water gaseous), but the quantity of heat liberated changes with the equation of decomposition. According to Berthelot,[A] the heat of formation of collodion-cotton is 696 cals. for 1,053 grms., or 661 cals. for 1 kilo. The heat liberated in the total combustion of gun-cotton by free oxygen at constant pressure is 2,633 cals. for 1,143 grms., or for 1 kilo. gun-cotton 2,302 cals. (water liquid), or 2,177 cals. (water gaseous). The heat of decomposition of gun- cotton in a closed vessel, found by experiment at a low density of charge (0.023), amounts to 1,071 cals. for 1 kilo. of the substance, dry and free from ash. To obtain the maximum effect of gun-cotton it must be used in a compressed state, for the initial pressures are thereby increased. Wet gun-cotton s much less sensitive to shock than dry. Paraffin also reduces its liability to explode, so also does camphor. [Footnote A: "Explosives and their Power," trans. by Hake and M'Nab.]