92 ANNUAL OF SCIENTIFIC DISCOVERY. 



pressed powder to be burnt before the flame reaches the charge in 

 a shell, the time of explosion may be adjusted with great nicety, 

 subject, however, to slight variations which depend, as Dr. Frankland 

 has recently shown, upon fluctuations in the density of the atmos- 

 phere. The production of rockets, signals, and numerous pyrotechnic 

 arrangements, is based upon the principles applied in these fuses. 



Although the gradual action of gunpowder is, as we have seen, of 

 the greatest importance in most applications, there are certain in- 

 stances in which more rapidly combustible substances, or more rap- 

 idly explosive bodies, undoubtedly might be employed with advantage. 

 This is particularly the case, for example, in mining operations, 

 where it is mainly desired to produce great destructive etfects by the 

 explosion. This circumstance has frequently led to the trial, and 

 even occasionally to the use, for a brief period, in actual practice of 

 bodies more readily and rapidly explosive than gunpowder. Only 

 one explosive compound, gun-cotton, has been put to the practical 

 test, but trials have been made of a variety of mixtures, in all of 

 which chlorate of potash is employed instead of saltpetre, in admix- 

 ture with very oxidizable substances. The preparation of any of 

 these substances upon a large scale has, however, always been sooner 

 or later attended by disastrous results, which in many instances have 

 not been simply due to carelessness. Examples of these mixtures are 

 Callow's mining powder, containing orpiment, or sulphide of ar- 

 senic, mixed with chlorate of potash, and German or white gun- 

 powder, which consists of prussiate of potash, chlorate of potash and 

 sugar. 



We have some of these mixtures which are so prone to change as 

 to be ignited instantaneously by contact with powerful chemical 

 agents, such as acids, for example. Chlorate of potash is most readily 

 acted upon by sulphuric acid, which not only decomposes the salt, 

 but also transforms the chloric acid into very unstable compounds, 

 and the heat resulting from the chemical changes thus established by 

 the acid in a small portion of the mixture of chlorate of potash with 

 an oxidizable body, such as sugar or sulphide of antimony, is suffi- 

 cient to ignite it, and thus the whole is almost instantaneously explo- 

 ded. Again, friction will ignite some of these mixtures very readily, 

 as by rubbing together in a mortar a few grains of chlorate of potassa 

 and roll sulphur. Even in the manufacture and employment of com- 

 paratively so safe an agent as gunpowder, which may be subjected 

 without ignition to tolerably powerful friction or percussion, and to 

 the direct application of any temperature below that which suffices to 

 ignite sulphur (about 550 Fahr.), the neglect of strict precautions 

 for excluding the possibility of a particle of the powder being sub- 

 jected to sudden and powerful friction, may, and frequently does, 

 lead to accidental explosions. The occasional accidents in gunpow- 

 der manufactories are generally enveloped in mystery, in consequence 

 of their fearfully destructive effects ; in all cases, however, where it 

 has been possible to trace the causes of such explosions, they have 

 been found in the wilful or accidental neglect of simple precaution- 

 ary measures, indispensable to the positive safety of the works and 

 operators. 



The more highly explosive mixtures, and some few explosive com- 



