46 POPE : CRYSTALLIME FORM OF IODOFORM. Form of Iodo form. By WILLIAM JACKSON POPE. UP t o the present, no geometrical measurements with any pretensions to accuracy have been made on crystals of iodoform, the only available data consisting of two angles measured by Rammelsberg (Kryst.-phys&%. Chm., 1882,2,321) ; these measurements cannot, however, be credited with much value since Rammelsberg remarks of his crystals that '' die Flachen sind ziemlich matt." Considerable difficulty is found in obtaining well developed crystals of iodoform using the ordinary organic solvents such as alcohol or benzene. Iodoform is, however, fairly soluble in acetone, and on allowing the cold solution t o evaporate spontaneously at n uniform temperature, magnificent, transparent, six-sided tablets of iodoform separate ; in order to obtain the best results, the acetone should be as free from water as possible.Even when pure, almost odourless iodoform is used, blacken- ing occurs round the sides of the vessel, and gome product, probably iodacetone, is formed during the evaporation which excites to tears, and powerfully affects the mucous membrane. The crystals, which may readily be obtained of a centimetre in dia- meter, and several millimetres in thickness, are uniformly developed, and the faces give very perfect reflections on measurement. The dominant form is c (1 1 l}, the pyramid (100, 22T) being very much smaller ; the prisms p (rl0) and 9n (21i) are rarely observed and are always small (see Fig. 1). The crystals are very hard and brittle, freePOPE ! CRYSTALLINE FORM OF IUDOFORM.47 Mean. from striations, and have no noticeable cleavage ; they show a six-sided internal growth of hexagonal symmetry when allowed to grow rapidly. The normal hexagonal optic axial interference figure is seen on 00110- scopic examination through the faces of c { 11 l} ; the double refraction is strong and negative in sign. Rammelsberg (Zoc. cit.) determined the angles cr (1 11 : 100) = 52' 0' and rr (221 : 100) = 4 6 O 30', gives the axial ratio u : c = 1 : 1.108, and only observed the forms c ( l l l } and r (100, 221). Crystalline system.-Hexagonal. a : c = 1 : le1O84. a = 93' 41'. Forms observed c (11 l}.. ............ .(0001} (ioo,22i}.. .... ...{ i o i i } p (110) ............ .,.{2110) {21i] ............ ...( olio) - - Calculated, FIG.1. The following angular measurements were obtained. Angle. c r = l l l : 100 rr= 100 : 012 r~ = 221 : GO1 I.r=T_00 : 010 pr=llO : 010 rr =22T : 100 W= 2 2 i : zoo Cplll : 1E rm= 100 : 211 Number of measurements. 38 16 47 15 22 34 19 13 18 Limits. 51'55'- 52" 7' 85 58- 86 6 7550- 76 6 93 54- 94 3 46 43- 47 2 46 20 - 46 25 133 32 -133 41 89 57- 90 4 37 54- 38 9 51 "59'5 0' 86 2 0 76 0 30 93 58 10 46 57 30 4.6 23 40 133 36 10 90 0 20 38 2 0 - 86" 2'20'' 76 020 93 57 40 46 58 50 46 24 0 133 36 0 90 0 0 38 010 After cautious melting under acover slip on a microscope slide, iodoform crystallises readily, giving broad, individual crystal flakes, the surfaces of which are parallel to c (111) ; the optic axis of negative double re- fraction emerges normally to the film surface. It is interesting to note that a large proportion of compounds of simple constitution crystallise in systems of high symmetry, such as the cubic or hexagonal. It may also be remarked that, whilst the molecule of carbon tetriodide has partial cubic symmetry and crystal- lises in the cubic system, the molecule of iodoform has partial hexagonal48 PEHKIN AND THORPE : fl&DIMETEYLGLUTARIC symmetry and crystallism in the hexagonal system. Many other similar analogies could be quoted, and although complications due to polymorphism frequently occur, it may be stated that a knowledge of the chemical constitutions of simple compounds often allowg of a safe prognosis of the crystalline form which those compounds assume. CHEMICAL DEPARTMENT, CENTRAL TECHNICAL COLLEGE, LONDOX.