30 PKESSUBE OF GASES. 



same with the empirical relations which may be deduced from 

 these indications. 



Now take an explosive substance of which the action is 

 extremely rapid (p. 24). In this case the pressure is equal to 

 a force of calibration corresponding to the half of the crushing, 



TT 



K + c, the constants retaining the same value as above. 

 2 



Here again, for one and the same substance, the high pressures 

 tend to become proportional to the crushing ; but the indica- 

 tions deduced from the calibration must be reduced by half. 



7. Thus the limiting value of the pressure reduced to the 

 unit of density of charge appears to be a constant; let it be 

 called/; then 



s being the pressure observed for a density of charge, A. This 

 constant is characteristic for each explosive substance, and may 

 be called specific pressure. It corresponds to one of the defini- 

 tions which has been given of the force of explosive substances, 

 viz. the pressure developed by unit weight of the substance 

 detonating in unit volume, 



8. Maximum Effort. It should be observed, however, that 

 the specific pressure does not represent the maximum effort 

 which an explosive substance can develop. In fact, this effort 

 is that of a substance detonating in a space entirely filled, that 

 is, in a space equal to its own volume. Now the latter only 

 corresponds to the specific pressure for a body of which the 

 absolute density equals unity. It will therefore be less for a 

 body of which the density is less than unity, as in the case of 

 gaseous mixtures and explosive gases, as well as of certain 

 liquids. On the contrary, it will be greater for all solid ex- 

 plosive substances known up to the present. 



It may be calculated, and in fact, from the preceding law it 

 is easy to estimate, the effort of a substance detonating in a 

 completely filled space, it being sufficient to multiply the 

 characteristic number of the pressures by the real density of 

 the pure substance. For instance, the density of mercury ful- 

 minate being equal to 4 ! 42, this body would develop a pressure 

 of about 27,000 kgms. per sq. cm. by exploding in its own 

 volume : an enormous figure, and higher than that of all known 

 explosives. 



9. Up till now, in the calculations of the specific pressure and 

 of the maximum effort, we have supposed that the explosive 

 substance is entirely transformed into gaseous products. But it 

 may happen that a portion of the substance keeps its solid 

 state, which is the case, for instance, with dynamite, a mixture 

 of nitroglycerin and silicious earth. The volume of the latter 



