386 CHEMICAL DISCOVERY AND INVENTION 



bullets, at the base of which is a bursting charge, which may 

 be gunpowder or a high explosive, while in the nose of the shell 

 is arranged the time fuse connected by a tube with the bursting 

 charge. This can be so regulated that the shell bursts in the air 

 at any desired point. Shrapnel, however effective against 

 troops in the field, does but little damage to earth works, wire 

 entanglements, and other defences. Hence for the latter purpose 

 high explosive shells are required. These consist of forged steel 

 with comparatively thin walls and a heavy bursting charge. The 

 explosive with which such shells are charged is usually one of 

 the products of nitration obtained by acting on one or other of the 

 constituents of coal-tar (see Chapter XX, p. 310) with strong 

 nitric acid. 



Phenol or carbolic acid mixed first with an equal weight of 

 strong sulphuric acid and the compound introduced gradually 

 into three times its weight of strong nitric acid gives trinitro- 

 phenol or picric acid. This is a lemon yellow crystalline substance 

 which has long been used as a dye for silk and wool. It melts at 

 122-5 C., and is a moderately strong acid, forming a variety of 

 salts with bases. 



Many of the picrates explode when heated or struck, but picric 

 acid burns quietly. When the fused acid is supplied with a 

 detonator it explodes violently, and it has been largely used 

 under the name lyddite, or melinite, for charging shells. Ex- 

 perience in the South African War showed that lyddite shells are, 

 however, somewhat erratic. 



Trinitrotoluene, T.N.T., is found to be more trustworthy, and 

 though its explosive force is somewhat less than that of picric 

 acid it is preferred on account of its stability, and being not an 

 acid but perfectly neutral it is not liable to attack the surface of 

 metals. 



Toluene is a colourless liquid which by the action of strong 

 nitric acid is converted successively into three nitro-compounds : 



C 7 H 8 toluene 



C 7 H 7 N0 2 mononitrotoluene 

 C 7 H 6 (N0 2 ) 2 dinitrotoluene 

 C 7 H 5 (N0 2 ) 3 trinitrotoluene or T.N.T. 



Trinitrotoluene is a yellowish crystalline powder with a 

 melting point about 79 C. When detonated by mercuric 

 fulminate it explodes with great violence giving a quantity of 



