  Molecules and particles make up $\sim 40 - 70\%$ of carbon in the
interstellar medium, yet the exact chemical structure of these constituents
remains unknown. We present carbon K-shell absorption spectroscopy of the
Galactic Interstellar Medium obtained with the Low Energy Transmission Grating
Spectrometer on the {\it Chandra} Observatory, that directly addresses this
question. We probe several lines of sight, using bright AGN as backlighters. We
make our measurements differentially with respect to the bright source Mrk 421,
in order to take the significant carbon K absorption in the instrument into
account. In the spectrum of the blazar 1ES 1553+113 we find evidence for a
novel feature: strong extinction on the low-energy side of the neutral C
$1s-2p$ resonance, which is indicative of scattering by graphite particles. We
find evidence for characteristic particle radii of order $0.1-0.15$ $\mu$m. If
this explanation for the feature is correct, limits on the mass of the
available carbon along the line of sight may imply that the grains are
partially aligned, and the X-rays from the source may have intrinsic
polarization.
