X rays and crystal structure . olecule which is associatedwith each point of the lattice. It is not necessary toenter into the examination here; the results willalone be given. The structure which is finallyarrived at by a study of the spectra is shown inFig. 36. A full account of the way in which it isworked out will be found in the original paper.* In Fig. 36, for the sake of simplicity, onlycalcium and carbon atoms are shown in their placesin the unit cell of the structure. For the purpose ofshowing- the arrangement of the oxygen atoms, thestructure is given as a series of layers perpendicu
X rays and crystal structure . olecule which is associatedwith each point of the lattice. It is not necessary toenter into the examination here; the results willalone be given. The structure which is finallyarrived at by a study of the spectra is shown inFig. 36. A full account of the way in which it isworked out will be found in the original paper.* In Fig. 36, for the sake of simplicity, onlycalcium and carbon atoms are shown in their placesin the unit cell of the structure. For the purpose ofshowing- the arrangement of the oxygen atoms, thestructure is given as a series of layers perpendicularto the trigonal axis, in Fig. 36B. The way inwhich these layers are taken from the unit rhombis sufficiently explained by the lettering of thefigures. * Froc. Roy. Soc, A, Vol. 89, p. 468, 1914. ANALYSIS OF CRYSTAL STRUCTURE 117 From Fig. 36, it is apparent that the arrangementof the planes perpendicular to the trigonal axis canbe expressed very simply. The planes are evenlyspaced, and they contain alternately calcium atoms. O (100) (III) i Cd CO, Ca CO, Ca CaCOO OCaCOO OCaCOFig. 36. Ca CO3 Mg CO3 Ca (Dolomite) and groups of the composition CO3. This arrange-ment is represented diagrammatically in Fig. planes containing the group of CO3 of weight(i2 + 3 X 16 = 60) are evenly spaced with the planescontaining calcium, of weight 40. 118 ANALYSIS OF CRYSTAL STRUCTURE The structure of each crystal of the calcite seriesis similar to that of calcite itself; and in particularthey all have a similar arrangement of planes per-pendicular to the trigonal axis. Moreover there isanother crystal which though of very different com-position imitates the calcite series so closely in itscrystalline shape and molecular volume that it hasalways been supposed to be built up in the sameway. This substance is sodium nitrate, NaNOs-An investigation of its reflection spectra shows thatthis similarity is indeed the result of similarity ofcomposition. As still another crystal of the samestructu
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