388 CHEMICAL DISCOVERY AND INVENTION 



spins round and releases the detonator so that on impact it flies 

 forward into the bursting charge and strikes a firing needle which 

 causes the explosion of the bomb. Before being dropped a pin 

 which holds the vanes is withdrawn and a fall of 200 feet suffices 

 to set the vanes spinning as described. 



The incendiary bombs used by the Germans consist of a shell 

 wound round with tarred rope and containing a quantity of resin 

 and other inflammable matter, in the midst of which is a charge 

 of "thermit" (p. 262) with usually a quantity of red phosphorus 

 at the bottom. In thermit advantage is taken of the very high 

 temperature produced by the combination of metallic aluminium 

 with oxygen. A mixture of fine powder of aluminium with oxide 

 of iron was introduced under this name about 1898 for the purpose 

 of welding together steel rails, repairing castings, or heating iron 

 bolts white hot. The mixture being packed round the object 

 to be heated is ignited by means of a piece of magnesium ribbon 

 which can be lighted by a match. The iron in the oxide is reduced 

 to the metallic state and remains when the action is over as a 

 fused mass. 



Enough has now been written to show the reader the general 

 character of the chemical mixtures and compounds employed 

 for military and naval use and for the peaceful purposes of the 

 miner. But the subject is a very extensive one, and those who 

 desire more technical information can only be advised to read 

 the article on Explosives in Thorpe's Dictionary of Applied 

 Chemistry. 



An interesting application of explosives to the purposes 

 of agriculture has attracted some attention during very recent 

 years, especially on the other side of the Atlantic. In new 

 countries land has often to be cleared of wood and sometimes of 

 masses of rock before it can be brought into cultivation. In 

 order to get rid of trees it has been the custom in past times to 

 burn them and leave the stumps to rot, before attempting their 

 removal. This necessarily occupies a good many years, and the 

 work is difficult and laborious. 



As soon as modern explosives became available the idea of 

 blowing up such obstructions naturally arose and has been put 

 into operation on a considerable scale. But latterly the use of 

 dynamite has been resorted to for the purpose of preparing holes 

 for planting fruit trees and for loosening the soil between trees 

 in orchards. As with every newly introduced practice there has 



