The critical-field theory of electrical breakdown in vacuum is shown to be applicable to all of the twelve metals and to the one alloy tested. In this theory the electric field at breakdown is predicted to be independent of gap spacing when the field considered is that existing in the neighborhood of cathode field-emission sites. This field is enhanced with respect to the average gap field by a factor which can be calculated from measurements of the field-emission current prior to breakdown. A vacuum tube has been employed for the experiment in which one electrode is movable and in which foils may be used for the electrode materials. The critical field for breakdown compares well with that calculated by others in the four metals for which previous data exist.