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Crystallo-Chemical Analysis

crystal, prism and angles

CRYSTALLO-CHEMICAL ANALYSIS. Professor von Federow published in the Zeit schrift f. Krystallographie (38, 321-490; 501 513-575) a method by which it is claimed if a few measurable crystals of any one of the over 10,000 recorded substances be subjected to a short, goniometrical measurement on the two circle goniometer, occupying at most two or three 'hours and only a few minutes, a reference to the tables will identify the sub stance of which the crystal is composed. To test this out samples were sent of many well crystallized substances, some of which had only been investigated previously by one authority. and in every one of these cases the substance, often an organic compound of considerable complexity, was identified.

While the fact that each chemical compound, unless isometric, possesses its own characteristic series of angles is well established, the orienta tion or setting up of the crystals has hitherto been to a great extent arbitrary; for instance, in an orthorhombic crystal any one of three set ups could be chosen and in the monoclinic and triclinic any number are possible. To under

stand von Federow's method requires a study of his theory of aparallelohedrala structure. The work involves the determining of a ((correct setting') based upon the crystal structure, as de termined chiefly by graphic methods, on the assumption that the most densely packed planes of the space lattice correspond to the best developed crystal faces.

Thereafter the task is to measure five or fewer characteristic angles and compare these with von Federow's tabulation for over 10,000 substances, the angles for a triclinic crystal being, for instance, 1, between two prism faces; 2 and 3, between basal pinacoid and prism faces; 4, between basal pinacoid and pyramid; 5, be tween PYramid face and one of prism faces. The entire tables are unpublished as yet.