Law of constancy of interfacial angles

[7] The law was also observed by Domenico Guglielmini (Riflessioni filosofiche dedotte dalle figure de Sali, Bologna, 1688),[8] but it was generalized and firmly established by Jean-Baptiste Romé de l'Isle (Cristallographie, Paris, 1783)[9] who accurately measured the interfacial angles of a great variety of crystals, using the goniometer designed by Arnould Carangeot and noted that the angles are characteristic of a substance.

[10][11] Carangeot was a student of Romé de L’Isle at the time of his invention of the basic crystallographic measuring instrument.

[12][13][14] A French crystallographer, René Just Haüy, showed in 1784[15] that the known interfacial angles could be accounted for if the crystal were made up of minute building blocks (molécules intégrantes)[16] that correspond approximately to the present-day unit cells.

The decrement of the layers is in the proportion of 2:1, which leads to a dihedral angle at the top edge pq of 126° 87′, closely corresponding to that of the empirical crystal, of 127° 56′.

[17][18] The phenomenon of the constancy of interfacial angles is important because it is an outward sign of the inherent symmetry and ordered arrangement of atoms, ions or molecules within a crystal structure.