The effect of irradiating a number of sensitive explosive crystals (such as lead azide, silver azide, cadmium azide, nitrogen iodide and silver acetylide) with high-speed particles has been studied. They were subjected to irradiation by electrons, by neutrons, by fission products and by X-rays. All these substances were exploded by an intense electron stream, but experiment shows that this is a thermal effect and is due to a bulk heating of the crystal. Nitrogen iodide is exploded by fission products, but this substance is anomalous. With the other substances interesting changes within the crystal are observed and these affect the subsequent thermal decomposition but no explosion results. The experiments show that, in general, the activation of a small group of adjacent molecules is not enough to cause explosion. The effect of crystal size on explosion is also studied. It is shown that if the crystals are heated they do not explode unless their critical size exceeds a certain value, which depends upon the temperature and upon other factors. Under the conditions of these experiments the limiting size for a number of explosives is of the order of a few microns. The work supports the earlier conclusions (arrived at from friction and impact experiments) that the necessary ‘hot-spot’ size is large on a molecular scale (
ca
. I0
-5
to 10
-3
cm in diameter); otherwise successful growth to detonation will not occur. Optical and electron microscopy provide some evidence that thermal decomposition takes place preferentially at dislocations inherent in the mosaic structure of the crystal.