C.W. Bunn to Lawrence Bragg

I.C.I. Ltd, Welwyn Garden City 1¼ pages, typescript

Thanks Bragg for agreeing to communicate their paper [on structur of polyethylene terephthalate] to the Royal Society. Defends his view that "for packing purposes the hydrogen atom behaves as a sphere of definite radius". Crystallographers have not taken Van der Waals' radii very seriously, because they are much more variable than bond lengths. His survey of Van der Waals' radii begins to show the origin of the variations. The apparant radius of an atom at room temperature depends on the thermal motions in the crystal and on which other atom it is in contact with. "It is probably a matter of polarity." Might be able to develop a scheme of Van der Waals' distances against polarities, analogous to scheme of bond lengths against bond orders. Would like to retain the wording on packing. The subject of Van der Waals' radii needs discussion somewhere but not in that paper.