530 CONCLUSIONS. 



hardly plays any part in the case of service powder, which 

 barely contains one per cent, of its weight of water ; but water 

 is, on the other hand, a very important factor in gun-cotton, 

 nitroglycerin, and in the majority of organic explosive sub- 

 stances. 



6. Having thus defined the volume of the gases we deduce 

 from it the pressure which they should exercise at the tempera- 

 ture developed by the explosion at constant volume, and even 

 at any volume. This calculation rests on the ordinary laws of 

 gases, laws whose application to these conditions requires the 

 greatest caution. Thus it is preferable, in practical application, 

 to measure the pressure of the gases direct from some of their 

 given mechanical effects, and particularly from the crushing of 

 small copper or leaden cylinders. 



The results should be referred to the weight of the water 

 contained in the unit of volume. Now, experience shows that 

 the pressure of the unit of weight for the unit of volume tends 

 to a constant value ; this is what we term specific pressure, and 

 this can be taken as a certain measure of force. Here we may 

 note a remarkable circumstance : the pressures found by experi- 

 ment are similar to the figures calculated by the ordinary laws 

 of gases, whether for solid or liquid explosive compounds ; at 

 least, for those which, in becoming transformed, give rise to pro- 

 ducts which cannot be dissociated, such as nitrogen sulphide 

 and mercury fulminate. 



On the other hand, in the case of gaseous explosive mixtures, 

 systems whose density for the unit of volume is low, we find a 

 considerable difference ranging from the single to the double, 

 and even beyond this. This difference may be attributable 

 either to dissociation, or to uncertainty as to the real laws of 

 gases, which would be applicable under these extreme con- 

 ditions. 



The maximum effort of an explosive substance evidently 

 applies to that case in which it explodes in its own volume. 

 Owing to this the effect will be all the greater in proportion to 

 the density of the substance. Such is the circumstance which, 

 added to the suddenness of the chemical decomposition, appears 

 to confer on mercury fulminate the pre-eminence over all other 

 bodies use^ as primings. The density of the fulminate is, in 

 fact, alrnostifive- times as great as that of nitroglycerin. This 

 allows mercury fulminate to exercise an effort which seems to 

 attain 27,000 kgm. per square centimetre, being almost triple 

 the effort exercised by the other known substances. 



Here we have the total consequences deducible from the 

 mere knowledge of chemical reaction. But in order to com- 

 pletely define an explosive substance it is also desirable to 

 know, as we have said above, what is the duration of its 

 transformation. 



