Ammonium ferrous sulphate and its alkali-metal isomorphs

Ammonium ferrous sulphate, (NH4 )2 Fe(SO4 )2 6H2 O, although one of the commonest double salts in everyday laboratory use, and noted for its excellent, comapratively stable, clearly transparent, plae greenish-blue crystals, has never yet been subjected to a through crystallographic and optical study. Since the year 1859, when a few of its principal angles were measured by Murmann and Rottter, and an approximate idea of its optical properties for red, yellow, and green light of no specific wave-lengths briefly indicated, just adequately to confirm that the salt belongs to the monoclinic series of double sulphates crystallising with 6H2 O, no accurate measurements have been made. The substance has, however, formes the subject of several special researches from a different point of view, such as those of von Hauer on the parallel growths of this salt on crystals of other salts of the series, of Baumhauer concerning the etch-figures produced on the various faces of the crystals by a small quantity of water, by St. Meyer on the nature of the crystals deposited under the influence of a magnetic field, by Wulff on the different rapidity of growth of the different crystal faces, and by von Fedorow concerning the correct setting of the crystals for descriptive purposes. But this salt was not included in the well-known series of optical investigations of Topsöe and Christiansen in 1874-5, nor the later one of Perrot, and has not been hitherto included by the author in his detailed investigation of the salt of this isomorphous series. This omission is now, however, removed by this communication.