NICOLAUS STENO 171 



from a solid in having its particles in constant motion and with- 

 drawing from their neighbors, that is to say, changing their relative 

 positions. 



Some of Steno's greatest contributions to science lie in the field 

 of crystallography, for he studied the growth of crystals and showed 

 that those formed in the mountains must have developed in the 

 same manner as crystals of niter separating from solutions in water. 

 These grow, he said, by accretions of substance upon the surface of 

 the crystal nucleus, and not as do plants and animals. 



The prevalent columnar form of crystals and the variation of their 

 habit through the occurrence of faces of variable size, Steno ex- 

 plained by the addition of substance on certain sides only of the 

 growing crystal. The force which draws the substance out of the 

 surrounding fluid he recognized to be inherent in the crystal itself, 

 and this crystallizing force he happily likened to what we should 

 to-day call the lines of force about a magnet. 



It is hardly to be expected that, great as Steno was, he should in 

 his day have discovered the important fact of the orientation of the 

 molecules of crystals, but he did point to the striking peculiarity of 

 light refraction that distinguishes the crystal from amorphous sub- 

 stances, such as glass. Steno was, however, the discoverer of the 

 fundamental law of crystallography known as the law of constancy 

 of interfacial angles. As usually stated, this law afifirms that no 

 matter how much the faces of a crystal may vary in their size or 

 shape, the interfacial angles remain constant, provided they are 

 measured at the same temperature. The absolutely empirical veri- 

 fication of this law was delayed until the invention of the reflecting 

 goniometer in 1805. Barring the refinement of temperature varia- 

 tions, it was amply verified by Rome de I'lsle with the simple goni- 

 ometer which he invented in 1783. It is clear, however, that Steno 

 more than a centuiy earlier fully grasped the principle of the law, 

 and gave it some sort of crude experimental verification. In the 

 explanation of his figures, Steno says (p. 272): 



" Figures 5 and 6 belong to the class of those crystals which I 

 could present in countless numbers to prove that in the plane of the 

 axis both the number and the length of the sides are changed in 

 various ways without changing the angles." 



As a corollary to his deductions concerning the growth of crystals, 

 Steno showed that so-called " phantom crystals " are no product of 



