SMALL thin red needles, heavily striated down the prism, were prepared by adding concentrated sulphuric acid to a cold concentrated solution of potassium dichromate. The crystals were sealed up separately in thin glass tubes to prevent deliquescence. Measurements on an ionisation spectrometer and from oscillation photographs gave the value of the cell size as a = 8.50 A., b = 4.73 A., c = 5.72 A., the probable error being not greater than ± 1 per cent. The density given by Groth is 2.73–2.82 and there are therefore 4 molecules per cell. The axial ratios given by Nordenskjöldl are a: b: c = 1: 0.69: 0.63. The relation between his ratios and ours is a: b: c (Wooster) = a: 3/4;b: c (Nordenskjöld). The only face from which he could obtain the ratio a: b should have been indexed (230) instead of (120). All the specimens which we have examined up to the present are mimetic twins on (110), closely simulating hexagonal symmetry.
A. E. Nordenskjöld, Pogg. Ann. Phy. u. Chem., 114, 622; 1861.
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