174 Mr. E. Threlfall on the 



viz. the action of detonators. The apparently anomalous 

 effects discovered by Abel may be summed up by taking the 

 most extreme case. Gun-cotton could be detonated by a 

 charge of fulminate of mercury, whereas ten times as much 

 nitro-glycerine was required to cause a similar sample of gun- 

 cotton to detonate. By firing the detonating charges on 

 copper plates, Abel naturally observed that the destruction 

 produced by the nitro-glycerine was much the greater, and 

 hence concluded that some other factor, besides the " explo- 

 sive violence," must come into play. This is undoubtedly 

 true, but the mistake arises in looking at the experiments from 

 one point of view alone, viz. that of the copper plate. There 

 will be no effect produced on the plate at all till the resistance 

 of the air becomes greater than that of the plate ; and this will 

 never be the case, however great the volume of gas liberated, 

 unless the time of explosion is sufficiently short. The re- 

 sistance of the air varies at least as the square of the velocity 

 of attack ; and therefore this will be the conditioning factor of 

 the destructive effect producible by explosions in free air. 

 For a given increment of volume occurring in an explosion 

 till the time of explosion diminishes to a certain value de- 

 pending on the strength of the plate, no effect will be ob- 

 served ; directly this limit is passed, the destructive effects 

 will depend, in the usual manner, on the quantity of energy 

 liberated. There is in fact a critical velocity of explosion, 

 below which the plate will not be attacked. But in a deto- 

 nation the case is different. We do not require any great 

 destructive effects, we only require that the time should be 

 so short that a portion, no matter how small, of the substance 

 to be detonated shall be raised to the appropriate temperature. 

 If the detonator has a time of explosion too great, then, 

 although the air may be the stronger obstacle and the ex- 

 plosive be destroyed, no detonation will be produced. This 

 is precisely what happened in Abel's experiments, where the 

 gun-cotton was blown to pieces by the nitro-glycerine. . The 

 instantaneous rise of pressure is not so great for nitro-glyce- 

 rine as for fulminate of mercury, though the energy run 

 down is much greater. This point has been satisfactorily 

 proved by Vieille in his experiments with the crusher-gauge. 

 Moreover the density of mercury fulminate is three times 

 that of nitro-glycerine, which allows a given mass to be on 

 the whole much nearer its work if it consist of fulminate of 

 mercury than if it consists ' of nitro-glycerine. 



We ought not therefore to be surprised that the detonation 

 of gun-cotton is easily accomplished by fulminate of mer- 

 cury, and hardly accomplished at all by nitro-glycerine. If 



