48 DURATION OF EXPLOSIVE REACTIONS. 



purpose of ensuring the regularity of the phenomenon, has been 

 found to be between 10 and 13 mms. per second for service powder. 

 It varies, moreover, in an inverse ratio to the apparent density 

 of the powder, in which the fire propagates itself in successive 

 layers. 



3. The velocity of combustion in this sense, that is to say, the 

 velocity with which the reaction is transmitted into the interior 

 of a single explosive mass, should not be confounded with the 

 velocity of inflammation, that is, with the time necessary for the 

 propagation of the same reaction in a whole formed out of a 

 series of small masses or grains placed side by side. In order 

 to characterise the difference between these two velocities, we 

 shall give the experiments made by Piobert with hollow iron 

 semi-cylinders of various lengths. The velocity of inflammation, 

 measured in the open air, varies in the old service powders 

 from 1*5 metre to 3'4 metres per second ; in tubes closed at one 

 end, from 0*30 metre to 1*5 metre, according as the grain is 

 0*2 mm. or 2*5 mms. in diameter. The increase is in propor- 

 tion to the size of the grain. These figures are very different 

 from those given above for the velocity of combustion in a 

 single mass. 



4. The combustion velocity of powder depends on the pressure 

 of air or of the surrounding gases. 



At the end of the seventeenth century, Boyle made some 

 experiments on the combustion of powder in a vacuum, and 

 observed that, under these circumstances, the grains of powder 

 projected on a red-hot iron fused with detonating. If we try 

 a certain number of grains at the same time, explosion results ; 

 doubtless because the conditions of local pressure are changed 

 round each grain which deflagrates. 



Huyghens repeated the same experiment by igniting powder 

 by means of a lens which concentrated the solar rays. 



If the heating is progressive, as would be obtained by the 

 radiation from a lighted coal, the sulphur may either be sub- 

 limed, thus destroying the homogeneity of the compound, or the 

 powder can be fused according to Hawksbee (1702). 



These experiments have been often tried with various modi- 

 fications, such as the employment of a platinum wire brought to 

 red heat by electricity, for the purpose of igniting the powder 

 in a vacuum (Abel). Bianchi has thus observed that gun- 

 cotton slowly decomposes in a vacuum before exploding, and 

 this result has since been extended to nitroglycerin by Heeren 

 and AbeL 



Mercury fulminate on the other hand explodes in a vacuum 

 when placed in contact with a brass wire heated to redness ; 

 but the explosion does not propagate itself to grains which are 

 not contiguous, as it would do under atmospheric pressure. 



5. Not only does a vacuum prevent the explosion of the powder 



