lO Petavel, High-P7'essure Explosions. 



One more point deserves attention. It will be noticed 

 that, 0'05 second after firing, the rate of pressure suddenly 

 increases, the rate of rise being over nine times as fast as 

 before. For the less explosive mixtures this change in 

 curvature does not occur, the curve of rise of pressure 

 being similar to the cooling curve, only of course much 

 steeper. It is worthy of note that the change of curvature 

 occurs when the gas is at a mean temperature about equal 

 to that at which spontaneous ignition would take place. 

 A similar result would therefore be obtained if we heated 

 the gases by the combustion of a certain portion of them 

 until the entire bulk was at the " flash point ; " the com- 

 bination would then take place simultaneously throughout 

 the entire mass, resulting in an almost instantaneous rise to 

 the maximum temperature and pressure. 



Explosives may be divided into three classes — instan- 

 taneous, medium, slow — according to the speed at which 

 the maximum pressure is attained. To the first class, 

 comprising fulminate of mercury, nitro-glycerine, etc., 

 the speed being equal to or above the natural period 

 of vibration of solid bodies, the term " pressure " hardly 

 applies, the phenomenon being not static but kinetic, of the 

 nature of an impact and not a pressure. If the explosion 

 takes place in a closed vessel, a certain pressure will follow 

 the combustion, but this pressure is no measure of the 

 stresses exerted. The second class would contain such 

 explosives as ordinary gunpowder, cordite, and mixture of 

 hydrogen and oxygen, the time of rise varying, 

 according to circumstance, from o'oooi to O'OI second. 

 Mixtures of air and coal gas, and the more diluted 

 mixtures of hydrogen and oxygen, take their place in the 

 third category, with speeds from ooi to i second. 



Before closing, it should be stated that the present 

 research has been greatly facilitated by funds awarded by 



