An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall.

  • Groth, P. (Paul), 1843-1927.
Date:
1906
Catalogue details

Licence: Public Domain Mark

Credit: An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall. Source: Wellcome Collection.

    47/144 (page 31)
    similarly1 ; undoubtedly polysymmetric (trigonal and pseudo- trigonal) is cerium silicotungstate, Ce4(W12SiO40)g,8i H20.2 It has already been stated on page 16 that, as a rule, that modification of a polymorphous substance which is stable at higher temperatures possesses the lower density, and such, as a matter of fact, is the case for the following among the examples already fully treated, namely-—Sulphur (page 17), mercuric iodide (page 19), and hexachlorethane (page 20) ; and, in addition, for quarz and tridymite, as also for a series of other substances for which the densities of the different modifications are known. On the other hand, there has been already mentioned (page 21) a sub¬ stance, ammonium nitrate, which shows the opposite character. It is true that the transformation of the first rhombic modification into the second rhombic one follows the above rule, but the transformation of the second rhombic modification into the trigonal one is accompanied by an increase of density. The same is the case with the transformation of trigonal silver iodide, Agl, into the cubic form, which is stable at higher tempera¬ tures ; also with dipropylammonium chloroplatinate, (NH2(C8H7)2)2PtClG, which, according to Ries, forms two monoclinic modifications of totally different crystal structure ; further, also, with the transformation of boracite into the cubic form (page 9), and of the mono- clinic calcium chloraluminate into the trigonal modification (page 9). Analogous cases, in which the modification corresponding to the higher temperature is likewise the denser, are those of arsenious oxide, As406 ; antimonious oxide, Sb4O0 ; also calcite and aragonite, CaC03 ; potassium calcium chromate, K2Ca(Cr04)2,2H20 ; and, among the ■elements, phosphorus and arsenic (whilst carbon is normal). Since, amongst the above-mentioned polymorphous sub¬ stances of the second kind, there are cases, such as those of 1 Zeits. f. Kryst. 1887, II, 303. 2 Wy rout off, Bull. soc. fr. min. 1896, 19, 219 et seq. ; Zeits. f Kryst. 29, 667.