arrangements of these integrant molecules in successive layers, according to geometrical laws of crystallization. Crystals that had been classed together previously were identified as being of separate mineral species if their fundamental structure differed. Heavyspar, for example, was differentiated into specimens containing barium and strontium. The value of Haüy's discovery was immediately recognized. Haüy and his contemporaries worked with limited evidence. They could observe a crystal's habit and cleavage planes and measure interfacial angles with an instrument called a goniometer. The internal structure underlying the crystal's integrant molecule would not be determinable until the development of X-Ray diffraction technology many years later, in 1902. Haüy was not the only researcher to observe that calcite crystals could be composed of smaller rhombohedra, but it was he who introduced the idea of triple periodicity in crystals. This idea was fundamental to later developments in the field on crystal lattices. Between 1784 and 1822, Haüy published more than 100 reports discussing his theories and their application to the structure of crystalline substances. Haüy first stated his laws of decrement in Essai d'une théorie sur la structure des crystaux (1784). It was a radical departure from his previous works, introducing his theory of molé constituantes or constituent molecules. By 1792, he had identified a number of parallelepipeds as possible primitive crystal forms. Haüy worked out the mathematical theory of his work in his Traité de minéralogie (1801), which became a classic in the field. By then, Haüy had applied his ideas to the differentiation of different species. He systematically described all the known minerals, sorting them into classes, and giving their chemical and geometrical properties. His work, in four volumes, including an atlas of plates, was accounted among the most wonderful of the 19th century. It has been described as "a work of comprehensive insight, and much of it, written with literary fluency". A second updated edition appeared as Traité de cristallographie in 1822. Haüy created comprehensive collections containing hundreds of pear-wood models of crystal models for education and demonstrations. One such set was acquired by Martin van Marum, curator of the Teylers Museum and a director of the Hollandsche Maatschappij der Wetenschappen. Haüy is also known for his observations on pyroelectricity. He detected pyroelectricity in calamine, an oxide of zinc, as early as 1785. He studied pyroelectricity in a number of other minerals including tourmaline and related them to crystalline structure. He showed