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We analyze Hubble Space T elescope Imaging Spectrograph spectra of two white dwarfs, Feige 24 and G191-B2B, and we compare the measured abundance patterns. Except for carbon, which appears overabundant in G191-B2B by 0.3È0.5 dex relative to Feige 24, the average heavy-element abundance in Feige 24 is 0.17 dex larger than in the cooler, hence older, G191-B2B, indicating a slow but perceptible decline of metallicity with time. We also found that the C IV j1550 doublet in G191-B2B is composed of adoi:10.1086/320653 fatcat:zidkq3mlnbhblpll3lo7v7pzzm