290 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1908. 



remains to be determined what minimum quantity of such combustible 

 can be added to avoid flames of great length and duration. 



The next question is, How can one tell whether an explosive is 

 " safe " ? This question is a still more difficult one to answer. The 

 various governments and also certain factories have erected testing 

 stations. These stations generally consist of a long wooden or iron 

 tunnel, round or oval in section. The explosive is shot into a gas 

 mixture. In this country a ballistic pendulum is used to ascertain 

 the quantity of the explosive equal in force to 4 ounces of dynamite 

 No. 1, and this quantity with a stemmed shot is then fired in air con- 

 taining 15 per cent of coal gas. If the mixture does not fire in 20 

 shots the explosive is considered a safe one. In most other countries 

 the quantity of the explosive in question is determined which will fire 

 and that which will just not fire a certain pit gas mixture. This 

 gives us what Mr. Watteyne, the well-know^n Belgian authority, calls 

 the " charge limite " of an explosive. This latter way is certainly the 

 more rational one, since it permits of comparison between difl^erent 

 kinds of explosives. Is this method of testing, however, above re- 

 proach? I think not, although I know of no better one at present. 

 It has been found that the narrower the bore of the cannon the easier 

 ignition takes place under certain circumstances. The AVoolwich 

 circular section gallery, which has a sectional area of 0.36 square 

 meter, is much more sensitive than the elliptical Belgian one, whose 

 sectional area is 2 square meters, and, in fact, even with equal diam- 

 eters each gallery may be said to have its own ignition temperament, 

 which affects the results. Thus quite recent tests at Frameries in a 

 gallery having a sectional area of 0.28 square meter showed that two 

 safety explosives, whose charge limite was 900 and 450 grams, re- 

 spectively, fired at 300 and at 75 grams. The gas used also exerts 

 considerable influence on the tests. 



It has been known for a long time that coal dust as well as pit 

 gas is highly explosive.' I believe that Engler, when investigating 

 explosions in the charcoal heaps of the Black Forest,* was the first to 

 show that mixtures of coal gas and air, so poor in gas as to be non- 

 inflammable, were rendered explosive by the addition of some char- 

 coal dust. The Mining Association of Great Britain took the lead, 

 experimentally investigating the influence of coal dust on explosions 

 in mines. An iron shell 7 feet G inches in diameter and 1,083 feet 

 long was used to carry out the experiments. So far it has already 

 been ascertained ^ that two zones of stone dust on either side of a zone 

 of coal dust arrested the path of a flame, and that unless the coal- 

 dust zone exceeded 180 feet in length, no explosive force was mani- 



aCbeuiiscbe Industrie, ]885, No. 6. 



^ " Coal dust experiments," The Times, September 24, 1908. 



