An X-ray study of some simple derivatives of ethane.—Part II

Tetrabromdimethylethane. CH3 . CBr2 . CBr2 . CH3 . M = 373·75. Two orthorhombic forms of this substance have been described in Part I of this paper. There is also, however, a stable tetragonal modification. Grown from ether or ligroin at ordinary temperatures, this modification shows only two forms, which Federow calls {111} and {110}. The latter are small and often absent. At the first attempt a number of large (about 3 × 3 × 4 mm.) bipyramids of this type were obtained overnight. Measurements on the Bragg spectrometer gave:— (111) : (110) = 28° 55' (Federow, 28° 50') (111) : (111̅) = 76° 29' leading to the axial ratioa :c = 1 : 1·280. The density found by the flotation method was 2·818 gr. /c. c. Unfortunately the absorption of such large crystals for Rh rays was too high for measurements to be made with any accuracy except for planes occurring as faces on the crystal. Moreover, the crystals were too soft to permit of any grinding of other planes. Only a very few measurements could be made, therefore, and these were insufficient for a reliable determination of the unit cell. All attempts to obtain Laue photographs of these large crystals were unsuccessful and when subsequent attempts were made to grow fresh smaller crystals, only the orthorhombic (b ) modification could be obtained. In fact, no subsequent recrystallisations gave the tetragonal form.