## Abstract

A consistent set of unit cell parameters at various temperatures is not yet available for ordinary ice, but the mean of the most precise measurements leads to a density of 0$\cdot $9164 g/cm$^{3}$ at 0 degrees C (atmospheric pressure) with a cubical expansion coefficient of 11 $\times $ 10$^{-5}$, increasing to 0$\cdot $9414 and 21 $\times $ 10$^{-5}$ at liquid air temperatures. Corresponding figures for heavy ice are 1$\cdot $0172 g/cm$^{3}$ and 12 $\times $ 10$^{-5}$ at 0 degrees C, 1$\cdot $0449 and 18 $\times $ 10$^{-5}$ at - 180 degrees C. The hydrogen-bond lengths are not significantly different for ordinary and heavy ice, but in both cases the mirror-symmetric bond (along the principal axis) is about 0$\cdot $01 angstrom shorter than the centro-symmetric bond at 0 degrees C. At low temperatures the bond lengths tend to equalize at a value some 1% lower than at 0 degrees C. The hexagonal (tridymite-type) and cubic (cristobalite-type) forms of ice have approximately the same density and hydrogen-bond lengths at - 130 degrees C, and both appear to have a statistical randomness of the water-molecule orientation, consistent with there being one hydrogen only (nearly or exactly) along each bond. The thermal vibrations of the hydrogen atoms in hexagonal ice are anisotropic, those of the oxygen atoms nearly spherical. The ranges of stability of hexagonal, cubic and amorphous ice are not exactly known, but cubic ice is only formed at low rates of deposition, low pressures and at temperatures of about - 80 to - 140 degrees C.