The hexahydrated double selenates containing thallium. Completion of the thallium salts and of the whole monoclinic series

The introduction to the preceding memoir (p. 367) applies equally to this paper, in which are described the six double selenates containing thallium as the R-metal and magnesium, ferrous iron, nickel, cobalt, manganese, and copper as the M-metal respectively. The optical and volume properties and constants of the zinc-thallium selenate are also included, as these were not determinable with the crystals described in 1909, while lately the author has obtained quite excellent crystals of this salt suitable for all purposes. While this work has been in progress a paper by L. C. Lindsley and L. M. Dennis has appeared, concerning five of these thallium double selenates, those in which the M-metal is copper, cobalt, nickel, magnesium, and manganese, which they consider to have made for the first time. This is, of course, an error, as all of them were made by the author previous to 1909, as will be clear from p. 367 of the preceding paper; but, as there stated, the crystals obtained were not of adequate perfection for complete goniometrical, optical and density measurements and determinations. Lindsley and Dennis, however, only give measurements of two angles, and these are supplementary to each other, being the acute and obtuse angles of the primary prismp {110}. They give no optical or other physical data. They found in the case of each salt an increase of about 40' in the acute angle of the prism, and a like amount of diminution of the supplementary obtuse angle, compared with the corresponding angle on the crystals of the analogous double sulphate.