An experimental study has been made of the physical conditions which occur during the impact of solids on liquid explosives. It is shown that the high impact sensitivity of liquid or gelatinous explosives is due to the entrapping of minute gas spaces during the impact. These tiny gas bubbles (the mass of bubble may be about 10$^{-10}$ g. and its volume at atmospheric pressure about 10$^{-7}$ c.c.) are heated by adiabatic compression and initiate the explosion. Bubbles of this size are, of course, difficult to detect, and they are readily entrapped under many conditions of laboratory experiment and practical operation. Because of the effect of the entrapped gas spaces the sensitivity of liquid explosives is profoundly affected by the shape of the impacting surfaces or by the distribution of the explosive. If, for example, one of the surfaces contains a small pin-hole or cavity, nitroglycerine may be exploded when the potential energy of the fall hammer is as low as 20 g.cm. If the liquid explosive is initially distributed on the anvil in the form of droplets or as parallel strips which coalesce during the impact and entrap gas, the sensitivity is also very high. Even when a continuous film of the explosive is struck between flat surfaces small amounts of gas may be entrapped and may initiate the explosion. If precautions are taken to prevent the entrapping of gas, nitroglycerine is comparatively insensitive to initiation by impact between curved or flat surfaces, and impact energies of the order of 10$^{5}$-10$^{6}$ g.cm. may be necessary to cause explosion. The effect of the gas is mainly a physical one (adiabatic heating), but the chemical nature (oxidizing property) of the gas is also important.

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