Crystals. ikj 



covery, which was more a deduction from the mathematical form of the 

 particular body he observed than a broad generalization from a series of 

 observations ofdifferent bodies. Itmust bebornein mind that the ancients 

 knew and had described crystals of certain minerals as having a constant 

 number of faces (or planes) arranged in a particular way. But Steno 

 went further than this and shewed that another constant existed. He 

 cut a number of sections of variously shaped prisms of quartz (r.) at 

 right angles to the edges of the prism, and (2.) at right angles to the edge 

 formed by a face of a pyramid with a face of the prism and found in the 

 first case (see Fig. 1 ) that the angles of any one section were equal to each 



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Fig. 1. 



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other and also to every angle of the other similar sections, and in the 

 second case (see Fig. 2) he found that the sections had two angles equal 



V 



Fig 2. 



lob anl four angles equal to r,except when the prism was absent in the 

 crystal, when the section was a four-sided figure with two opposite angles 

 equal to b, as shewn on the left in Fig. 2. 



His inference was that in all specimens of Rock-crystal correspond- 

 ing pairs of faces have the same inclination. 



Thus was taken the first step towards the discovery of one of the 

 three great fundamental laws governing the formation of crystals, which 

 has been enunciated thus : 



the law of CONSTANCY OF ANGLES. Crystals of the same substance, 

 whether natural or formed in the laboratory, are essentially constant in 

 the angle of inclination between like planes. 

 Fora whole century the law discovered by Steno was not elaborated until, 



