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.

    37/144 (page 21)
    Ammonium nitrate, NH4NO;i, according to Lehmann’s investi¬ gations,1 between the ordinary temperature and its melting point passes through the limits of stability of no less than four modi¬ fications ; consequently, it is “ tetramorphous.” The dry salt melts at 1680, the moist salt somewhat lower, since it dissolves in a very small quantity of water; in both cases it solidifies to optically isotropic crystal skeletons. At 125-6° the mass suddenly becomes doubly refracting, and in the case of a solution optically uniaxial crystals are formed (rhombohedra). If the solution cools still further, then, at 82-8° there are produced, from the growths of the former modification, needle-shaped rhombic crystals, orientated in definite directions with regard to the preceding growths. Finally, at 32-4°, these rhombic crystals become transformed into a fourth modification which is also rhombic ; this is the one which is obtained in large crystals from aqueous solutions at ordinary temperatures. If this last modification is heated it becomes successively transformed into the three other modifications at the appropriate temperatures. Bellati and Romanese determined the changes in the densities and in the specific heats at the transition temperatures,2 and Schwarz3 gave a more exact determination of the transition temperatures ; his values have been inserted above in place of those originally found by Lehmann. Bellati and Romanese found for the mean specific heats From o°- 32° (first rhombic modification) . . . 0-407 „ 320- 83° (second „ „ ) • 0-335 ,, 83°-i24° (trigonal ,, ) - 0-426 and from their determinations Schwarz calculated the densities and their reciprocal values—the specific volumes—of the three modifications, as follows :— First rhombic modification at 32‘J d— 1 -6560 Sp. Vol.'—o-6039 Second ,, f at 32 ” (at 83 i-6o2i I-5875 0-6241 0-6299 Trigonal ,, at 83 1.6093 0-6214 On the transformation of the last of these modifications into 1 Zeits.f Kryst. 1877, I, 106 ; Molekularphysik, I, 155. 2 Atti. 1st. Sc. Lett., etc., Venice, 1886, [6] 4, 1395 ; Zeits. f. Kryst. 14, 78 ; Journ. C. S. 54, 106. 3 Beitrage zur Kenntniss der polymorphen Korperf