EXPLOSIVES 387 



black smoke, whence some of the names Black Maria or Coal 

 Box given by the soldiers to shells of this kind. 



T.N.T. is sometimes mixed with other substances, especially 

 with an oxidising compound such as ammonium nitrate, together 

 with a little aluminium powder and a trace of charcoal, the 

 mixture being known as ammonal. 



Other constituents of coal-tar yield explosive compounds 

 under the action of nitric acid. 



Dinitrobenzene, for example, enters into the composition of 

 the mining explosives roburite and bellite. Trinitrocresol has 

 been used in place of picric acid under the name ecrasite, but it 

 shares the disadvantages of picric acid. 



Cheddite is a name given to a permitted explosive containing 

 potassium chlorate mixed with mononitronaphthalene, dinitro- 

 toluene, and a little castor oil. Another variety of cheddite 

 contains ammonium perchlorate. 



Probably the most powerful explosive known is tetranitro 

 aniline, and another similar compound tetranitromethyl aniline, 

 known as " tetryl," is already used for detonators in place of 

 mercuric fulminate. Another compound which has recently 

 found application as a detonator is lead hydrazoate or triazide, 

 PbN 6 , derived from hydrazoic acid or azoimide HN 3 . The acid 

 itself when in the pure anhydrous state and some of its organic 

 derivatives are among the most dangerously explosible com- 

 pounds known, as they sometimes explode violently without 

 obvious cause. But several of the metallic salts, such as the lead 

 salt mentioned above, and the barium salt, are fairly stable and 

 can be manipulated without risk, if proper precautions are taken. 



In blasting operations gunpowder and detonators are fired by 

 a time fuse or electrically. The time fuse is a case containing 

 gunpowder which is made to burn at a known rate, generally 

 2 feet per minute. The instantaneous fuse which burns at the 

 rate of 100 to 300 feet per second affords the means of firing 

 many charges simultaneously. 



Of the bombs which have come into use in warfare with the 

 development of airships and aeroplanes there are several varieties. 

 The British air service during the war is understood to have made 

 use of the bomb designed by Marten Hale, Fig. 143. This is an 

 ingenious arrangement which has the great advantage that it 

 can be handled and transported quite safely. In the neck of the 

 bomb is a propeller which, when free and falling through the air, 



