NICOLAUS STENO (1638 - 1686). De Solido intra Solidum. Florence: Ex Typographia sub Signo Stellæ, 1669.

Stensen attended the University of Copenhagen and then went on to Amsterdam, receiving an M.D. in absentia from the University of Leiden in 1664. Stensen became professor of anatomy at the University of Padua in Italy, where he was appointed house physician to Grand Duke Ferdinand II of Tuscany (1610 - 1670). During this decade he made his greatest contributions to science. In 1672, he returned for a time to Denmark, but came back to Italy in 1674.

The greatest contribution to crystallography, paleontology, and geology made during the seventeenth century was Stensen's De Solido Intra Solidum. Within this volume, intended as an introduction to a much larger but never published work, Stensen observed for the first time the fundamental crystallographic law of the constancy of interfacial angles. In drawings and two brief sentences, Stensen remarked that although crystals of quartz (silicon oxide) and hematite (iron oxide) come in a great variety of shapes and sizes, the same interfacial angles recurred in every specimen. The law of constancy of angles was confirmed and shown to be true for crystals of many other substances by Romé de l'Isle in 1783.

Stensen went on to discuss the growth of crystals in a fluid medium, but for him, this was only a special case illustrating the main problem of the book: how are solids of all kinds formed in nature? His answer was, "if a solid body has been produced according to the laws of nature, it has been produced from a fluid ... either immediately from an external fluid, or through one or more mediating internal fluids." Internal fluids accounted for the growth of animals and plants; sedimentation, incrustation, or crystallization from external fluids explained the formation of rocks and minerals. Stensen's observations on the growth of crystals are important to crystallography, but in addition the parallels between animal or plant growth and the formation of minerals led Stensen to recognize for the first time that fossils were the remains of dead animals.

The plates in De Solido Intra Solidum show different crystal forms and an illustration of stratifications of the earth's crust. The plate was originally created as a single large diagram, but due to its large size it is common to find it bound into the book as two separate plates.

Schuh, Curtis P. Mineralogy & Crystallography: An Annotated Bibliography of Books Published 1469 through 1919. Tucson: privately published, 2005, p1381-1382.

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