APPENDIX. 551 



this will take place at a moment which will vary with its own 

 instantaneous resistance. This resistance is quite different from the 

 static resistance of the vessel, which can be measured by hydraulic 

 pressure. 



Let us examine what actually happens when an explosive 

 detonates in a tube, the detonation being provoked in the first 

 instance by the violent shock of the mercury fulminate, which 

 immediately raises to the extreme limit the initial pressure, the 

 heat which it disengages, and the chemical reactions developed from 

 layer to layer which arise from it. 



No regular state of affairs corresponding to the explosion of the 

 matter in its own volume can be established, since the tube is 

 necessarily broken. However, if it be homogeneous, so that the 

 pressures and reactions can be propagated in a uniform manner, 

 then the tube will be regularly and progressively ruptured in pro- 

 portion as the pressure propagated attains a certain limit, and 

 thus a special regime of detonation may be established which will 

 depend on the conditions realised in the system. A velocity 

 of propagation fairly uniform for each given system will then be 

 observed, but very variable between different systems even when 

 the same explosive has been used, as shown by the experiments 

 with methyl nitrate and the tubes of different material. 



This regime of detonation depends on the structure of the 

 explosive as well as on the nature of the envelope. Thus nitro- 

 glycerin gives a lower velocity than dynamite, it being a viscid 

 liquid which transmits the shock which determines detonation 

 more irregularly than the silica uniformly impregnated with it. 

 Dynamite made with mica gives still higher velocities, which is 

 accounted for by the crystalline structure of the mica, this body 

 being more rigid than the amorphous silica. This view is also 

 confirmed by the observations made with nitromannite, a solid 

 crystalline body, which appears more apt to transmit the detona- 

 tion than liquid methyl nitrate, having given a velocity of 7700 m. ; 

 picric acid, another crystalline body, has given 6500 m. This 

 contrast between liquid methyl nitrate and crystallised nitro 

 compounds is thus in accord with what has been observed between 

 nitroglycerin and dynamite. 



On the other hand, in certain pulverulent systems in which 

 complete continuity has almost been attained by compression, 

 experiment has proved that there is a limit of compression beyond 

 which the mass cannot be exploded by a fulminate detonator. 

 This has been observed with certain powders formed of potassium 

 chlorate and tarry materials. 



A few further observations with gun-cotton may be given as 

 showing the influence of the envelope. 



Compressed gun-cotton, density of charge 1 and 1*27, gave 

 velocities of 5400 in. in leaden tubes of 3'15 mm. internal 

 diameter; while with density of charge 0'73, in a leaden tube 

 3*77 mm. internal diameter, the velocity observed was 3800 m., 

 the inequality being evidently due to the less continuity of the 

 material. The feeble resistance of the envelope may be com- 

 pensated by the mass of the explosive, which prevents, especially 



