18 MODERN SCIENCE READER 



inch. A more practical illustration is seen with nitro- 

 glycerin, which Nobel found yielded about 1,200 times its 

 own volume of gas calculated at ordinary temperatures and 

 pressures, while the heat liberated expands the gas to nearly 

 eight times its normal volume. 



Clearly, tfyen, a substance for use as an explosive must 

 be capable of undergoing rapid decomposition or combina- 

 tion with the production of large volumes of gas, and 

 further produce sufficient heat to greatly expand these 

 gases, the ratio of the volume of gases at the moment of 

 explosion to the volume of the original body largely 

 determining the efficiency of the explosive. 



Explosives may be divided into two great classes me- 

 chanical mixtures and chemical compounds. In the former 

 the combustible substances are intimately mixed with some 

 oxygen-supplying material, as in the case of gunpowder, 

 where carbon and sulphur are intimately mixed with 

 potassium nitrate; while guncotton and nitroglycerin are 

 examples of the latter class, where each molecule of the sub- 

 stance contains the necessary oxygen for the oxidation of 

 the carbon and hydrogen present, the oxygen being in 

 feeble combination with nitrogen. Many explosives are, 

 however, mechanical mixtures of compounds which are 

 themselves explosive, e. g., cordite, which is mainly com- 

 posed of guncotton and nitroglycerin. 



Two methods are in common use for bringing about 

 explosions ignition by heat, thus bringing about ordinary 

 but rapid combustion, molecule after molecule undergoing 

 decomposition ; and detonation, where the effect is infinitely 

 more rapid than in the first case ; in fact, it may be regarded 

 as practically instantaneous. The result may be looked 

 upon as brought about by an initial shock imparted to the 

 explosive by a substance the detonating material which 

 is capable of starting decomposition in the adjacent layers 

 of the explosive, thus causing a shock to the next layer, and 

 so on with infinite rapidity. That the results are not en- 

 tirely due to the mechanical energy of the liberated gas 



