a mixing capsule assembly , particularly for use in preparing dental amalgams , comprises an outer capsule for receiving one of the materials to be mixed , and an inner capsule to receive a second material to be mixed and to be enclosed within the outer capsule . the inner capsule includes two sections attachable together by complementary frictionally - engaging surfaces of conical configuration . the arrangement is such that when the inner capsule is impacted against an inner face of the outer capsule during the vibration of the assembly , the complementary surfaces are moved away from each other to permit some of the second material to pass between them and to mix with the material in the outer capsule ; and when the inner capsule is impacted against the opposite inner face of the outer capsule , the complementary surfaces are moved back to block the passage of material between them .