The Crystal Structure of Ice.

Crystal Structure of Ice as Determined from $X$-ray Pattern.—A small sample of ice composed of minute crystals, gave by Hull's method a pattern of 12 clearly defined lines whose positions indicate that the lattice of an ice crystal corresponds to a hexagonal close-packed arrangement of molecules, consisting of two sets of interpenetrating triangular prisms with sides 4.52 A and height 7.32 A. Since for close-packed spheres the axial ratio should be 1.633 instead of 1.62, the ice molecules act like spheres which have become flattened by 0.8 per cent. in the direction of the hexagonal axis. This lattice is practically the same as that found for magnesium, but the differences in relative intensity between the lines of the two X-ray patterns suggest that ice molecules and magnesium atoms must differ considerably in shape.

Molecular Formula for Ice.—From the density of ice and the dimensions of the lattice it follows that the formula for an ice molecule is ${\left({\mathrm{H}}_2\mathrm{O}\right)}_2$ or ${\mathrm{H}}_4$${\mathrm{O}}_2$.