An original ball-drop impact apparatus has been constructed and used to test the sensitivities of dextrin lead azide and mercury fulminate to mechanical impact. Ball masses and heights were both varied. Both the net kinetic energy and the change of momentum gave continuous functions with the percentage of detonations. Comparisons of the absolute values of the net kinetic energy and the change of momentum with another reported in the literature suggest that momentum is the more important factor in determining the probability of detonation. Values for the momentum at 50% detonations for lead azide and mercury fulminate are 4.6 × 104and 2.6 × 104 c.g.s. units, respectively, with an area of contact of about 0.08 cm2. The corresponding times of impact were found to be about 2.2 × 10−4and 1.9 × 10−4 sec.