THE PRODROMUS 237 



CONCERNING THE CRYSTAL 



As regards the formation of crystal, I would not venture to 

 declare in what manner its first shape is produced ; this at any 

 rate is beyond dispute, that the things which it has been my 

 lot to read in other writers concerning this subject are not to 

 the point ; for neither irradiations, nor a shape of the particles 

 resembling the shape of the whole, nor the perfection of the 

 hexagonal form ^ and the assembling of the parts about a com- 

 mon centre, nor other things of this kind, accord with fact ; as 

 will be clear from various propositions which I shall bring 

 forward, proved elsewhere by conclusive experiments. But 

 that no room may be left for doubt, it is well to explain before- 

 hand the terms which I employ in naming the parts of a 

 crystal. 



A crystal consists of two hexagonal pyramids and an inter- 

 mediate prism likewise hexagonal. I call those angles the 

 terminal solid angles which form the apexes of the pyramids, 

 but those angles the intermediate solid angles which are formed 

 by the union of the pyramids with the prism. In the same way 

 P. 38. 1 call the planes of' the pyramids term.inal planes, and the 

 planes of the prism the intermediate planes. The plane of the 

 base is the section perpendicular to all the intermediate planes; 

 a plane of the axis is a section In which lies the axis of the 

 crystal, which consists of the axes of the pyramids and the axis 

 of the prism. 



The place where the first hardening of a crystal begins, 

 whether it be between a fluid and a fluid, or between a fluid 

 and a solid, or even in a fluid itself, may remain in doubt ; but 

 the place in which the crystal grows after it has already begun 

 to form, is a solid in that part where the crystal is supported on 

 it, whether the place be a stone or another crystal already 

 formed. The remaining portion is fluid, if you except the 

 obstructions which can present themselves to it from the 

 unevenness of the rock or even from other crystals already 

 formed. I would not venture to affirm whether the surround- 

 ing fluid is aqueous ; and it matters not what is said about the 



1 By crystal Steno meant rock crystal, which is the mineral quartz and has a hexagonal 

 form. 



