The period of impact, the time of initiation and the rate of growth of the explosion of nitroglycerine

Electrical and photographic methods have been used to measure the period of impact between colliding solids and to study the time of initiation and the rate of growth of an explosion produced by the impact. The measurements were made with liquid explosives, where, as the last paper has shown, the initiation is due to the entrapping of small gas bubbles. Under the conditions of these experiments, the period of impact—that is the time from the first moment of contact of the solid surfaces until they separate orf the rebound—is about 200μ sec. If one of the surfaces contains a cavity, initiation with nitroglycerine is first apparent as a tiny spot of light inside the cavity. This occurs at the first moment of contact or a few microseconds after. This small flame spreads slowly through the cavity with a velocity of about 20 m./sec., and after a short interval of time (about 50μ sec.) bursts through the cavity wall and explodes the main film. The explosion is propagated through the film with a velocity > 1000 m./sec., and the residual burning inside the cavity is finally extinguished when the surfaces separate. When the initiation is due to a bubble entrapped in the coalescing liquid, the measurements show that initiation again begins as a small flame which becomes visible at the moment of coalescence or a few microseconds afterwards. Both the coalescence and the initiation can occur when the solid surfaces are still separated by a film of explosive. The flame is propagated outwards with a circular symmetry at a velocity of about 400 m./sec., and after a short distance it transforms into a higher velocity and is propagated through the thin film of explosive at a velocity > 1000 m./sec. These results support the conclusions of the last paper that the explosion is initiated at a small gas bubble which has been heated to a high temperature by adiabatic compression. This bubble of hot gas brings about the thermal decomposition of the explosive: the reaction begins as a comparatively gentle burning which, after a few microseconds, passes over into a more violent explosion.