On Cry st allograph]/, 121 



much too small to admit of its being a convenient measure 

 of temperature. 



I should not despair, however, (availing myself of every 

 advantage, viz. increasing the length of a metallic wire, 

 by giving it a spiral form, in order to comprise a consi- 

 derable length in small compass ; with the application of 

 the lever-index, and a good magnifier.) of constructing a 

 thermometer upon this principle, so as to render the scale 

 apparent even to single degrees; using silver for the lower 

 temperatures, and platina for the higher, or employing 

 iron wire, only up to its ultimate point of expansion in a 

 solid state *. 



I am, sir, 



Your obedient servant. 



Queen-street, Oxford, PvICHARD WALKER. 



Aug. 6, 1810. 



XX. On Crystallography. By M. Hauy. Translated 

 from the last Paris Edition of his Traite de Mineralogie. 



[Continued from p. 69.] 



THEORY OF THE LAWS TO WHICH THE STRUCTURE OP 

 CRYSTALS IS SUBJECTED. GEOMETRICAL PART. 



Preliminary Notions, 



1. JL he theory which I here propose to submit to calcu- 

 lation has for its object, to determine all the different forms 

 which may arise from a superposition of decreasing laminae 

 following known directions and laws, on the various faces 

 of a solid, the figure of which is also given f. 



2. The solid which I call nucleus or primitive form is 

 always one of the six following: 1st, the parallelopipedon; 

 2d, the regular hexahedral prism ; 3d, the rhomboidal 

 dodecahedron; 4th, the octahedron ; 5th, the tetrahedron, 

 which in this case is always regular; 6th, the bipyramidal 

 dodecahedron. 



3. By subdividing each of these solids parallel to its dif- 

 ferent faces, and sometimes also in other directions, we 

 obtain the integrant molecules, which are aKvays either 

 parallelopipedons, triangular prisms, or tetrahedrons. < 



* For the means of rendering exceedingly minute divisions distinct, see 

 a. method described in the Monthly Magazine for May 1810. 



f I presume that my readers are acquainted with that part of my treatise 

 jn which the same theory is detailed by simple reasoning. I shall there- 

 fore now confine myself to resuming in a succinct manner the most general, 

 principles of this theory. 



4. When 



