Measurements of the voltage required to produce discharges in voids within a number of different dielectrics show that the discharge-inception voltage across the void is independent of the nature of the dielectric and of the void position, including the case in which the void is bounded by one of the electrodes, but depends upon void depth or thickness in the direction of the applied electric field.The measurements, including those made at pressures and temperatures other than n.t.p., are consistent with Paschen's law for metal-to-metal breakdown of gas.