[en] Observations indicate that N (H2)/N (Na I) is relatively constant among clouds, while N (Na I)/N (Ca II) is variable. We use this result to argue that the gas-phase abundance of calcium differs among clouds because varying amounts of calcium are contained within grains. We also discuss the well-known enhancement of N (Ca II)/N (Na I) with cloud velocity, and we suggest that at least part of this effect can be understood with a model where grains are destroyed in grain-grain collisions