WHEN, in 1907, Einstein published his work on the specific heat of solids, nothing was known of X-ray crystallography. Crystals were apt to be regarded as a highly specialized form of solid matter. Now, as a result of the wide application of X-ray analysis to all solids, the crystalline state is known to be the normal stable state of solids. Theoretically, the crystal is a periodic three-dimensional grouping in space of an immense number of atoms, ions or molecules held together by interatomic or inter-molecular forces. The unit cell, the smallest portion of the crystal to contain all the groupings of atoms characteristic of that particular form of the substance, is very much smaller than the smallest visible crystal. Nevertheless, the strength of diamond and the greasiness of graphite are inherent in the two different patternings of carbon atoms in the two different unit cells.