Compressed Air Work And Diving 1909

98 COMPRESSED AIR WORK. the hammer, as the shock cannot travel through the paper. In 1866 Alfred Nobel invented dynamite by absorb- inQ- nitro-Q-lycerine in kiesel<juhr. This is a fine cliatomaceous earth consisting of the microscopic shells of diatoms mixed with organic substance. It is first of all burnt to get rid of all the organic substance, and then crushecl until it becomes a fine powder. This powder will absorb three times its own weight of nitro-glycerine, and is usecl in the manufacture of dynamite. Dynamite, therefore, contains 75 per cent, of nitro-glycerine, and is a very powerful explosive. It is safe to handle, and may be even burnt in small quantities without exploding; if, however, it is burnt in large quantities, or in sucli a way that its tempera- ture is raised beyoncl a certain point, it will explode. It is not suitable for use under water, because the nitro- glycerine will be caused by the water to exucle from the absorbent. Its peculiar characteristic is the instan- taneousness with which it explodes ; this makes its action local in character and radial in direction. In this respect dynamite may be saicl to stand at one end of the scale and ordinary sporting powder at the other. Dynamite acts all round, and gunpowder in the direction of least resistance. The difference is, however, one of degree rather than of kind. For this reason dynamite is very suitable for “capping” a boulder. When this is done, the charge is placecl 011 the boulder and covered by a piece of clay. The dynamite, if the charge is big enough, will crack the boulder ; gunpowder expancl- ing slowly will merely lift off the lump of clay. Nobels biasting gelatine is a still more powerful explosive than dynamite, but it explodes rather more slowly, so that as regards local action it is slightly lower