CONDITIONS OF EXPLOSION. 377 



In the absence of these conditions, which are not very 

 suitable for precise comparisons, trials are made on a small 

 scale, such as 



The use of the testing mortar on ballistic pendulum for powders 

 intended to throw projectiles from firearms ; 



The use of bombs of different thicknesses from which the 

 bursting charge (p. 58) and the mode of fragmentation are 

 studied ; 



The rupture of freestone, rails, T-iron, iron girders, masses of 

 rolled, cast, or wrought iron, beams of different kinds of wood, 

 and different scantlings, by charges laid on their surface ; 



The curve imparted to thick iron plates in comparative 

 conditions ; 



The crushing of a small block of lead by a charge placed on 

 its surface, with or without tamping ; 



The crushing of a copper cylinder (p. 20) ; 



The form and size of the chambers produced in a mass of 

 clay or lead by the explosion of an internal charge (see p. 

 374), etc. 



We shall refer to the technical treatises and memoirs for the 

 description of these various tests, as it would be almost impos- 

 sible to give the exact theory of them at present. 



5. The explosive substance should be capable of being handled 

 and transported by road or railway with at least relative safety, 

 and it must not be too sensitive to shocks or friction. This is 

 the reason why pure nitroglyeerin and chlorate powders are 

 almost excluded in practice. 



The same circumstance forbids the employment of dynamite 

 and pure gun-cotton in warfare, since these substances explode 

 from the shock of a ball. 



6. The substance should only explode in conditions which are 

 precisely known, and capable of being produced or avoided at 

 pleasure ; for instance special ignition, the use of certain caps 

 and fuses ; the employment of electricity to heat a wire or produce 

 a spark; the shock of two metal pieces arranged beforehand; 

 definite chemical reaction for instance, that of sulphuric acid 

 on potassium chlorate mixed with a combustible body, etc. 



The conditions under which the explosive substance is 

 brought to explode should be realisable without too much 

 difficulty ;' thus the explosion of paraffined gun-cotton becomes 

 almost impossible above a certain quantity of paraffin. In the 

 same way a mixture of petroleum spirit and nitric peroxide in 

 equal volumes does not explode under the influence of an 

 ordinary fulminate cap, while it does so by the addition of a 

 tenth part of carbon disulphide, etc. 



7. The explosion should produce effects foreseen beforehand, 

 at least in a certain limit, such as direction, general characters, 

 and intensity. 



