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We explore the incorporation of the cosmically abundant species O, C, N, Mg, Si, Fe and S into interstellar dust. Column densities based on Goddard High Resolution Spectrograph 3.5 km s$^{-1}$ resolution measurements from the literature for 8 individual absorbing regions toward 5 lines of sight are used. Assuming cosmic reference abundances based on solar and B star values, depletions and dust-phase abundances are derived which suggest that silicates, oxides, and possibly graphite are important constituents of the grain core population. Tha ratio of Mg to Fe atoms mantled on these grain cores is 1.8, whereas approximately 4.0 Si atoms are in the mantles per Fe atom. The abundances of O and N in the dust phase as implied by the solar reference abundance values are difficult to reconcile with the fact that these elements are not expected to participate in mantle formation, and the 3.1$\mu$m H$_{2}$O ice feature is not seen in absorption toward stars similar to those studied. The B star references for O and N, however, imply that no mantling of these species has occurred. The dust-phase abundance for C implied by the solar reference abundance agrees with predictions for the number of graphite grains needed to produce the 2175\AA\ extinction bump. B star reference abundances, however, suggest that the abundance of C in the dust phase is not sufficient to produce the bump.
UJS and BDS acknowledge support for this work from the NASA grant NAG5-1852. JAC acknowledges support from NAGW-2520.
