The microcanonical partition sum Z ( E ) of a heat bath of temperature T has the property Z ( E + Δ Δ E ) = Z ( E ) e Δ Δ E / k B T , where k B is the Boltzmann constant. It thus changes by the same factor when a given amount of energy is added. The exponential factor in this expression can be identified with the reciprocal of the Boltzmann factor.