Cryjlals of Oxidated Tin Ore. 353 



In the annexed drawing (fee plate III.) all the plans, and 

 alfo the figures marked ia, \b, and i c, are geometrical : the 

 Others are drawings from models, and in perfpeftive as they 

 appear to the eye; but all of them placed in the fame rela- 

 tive fituation to the nucleus, fig. i., on which they arefiip- 

 pofed to be formed. Mathematical correftnefs is not at- 

 tempted, nor is it necelfary. The prifms are fhewn with the 

 pyramid on one end only ; not on both, as in De I'Ifle's plates. 

 Fig. I, a perfpcftive view of the primitive oftaedron of oxi- 

 dated tin ore. 1 a, z front geometrical elevation, with the 

 quantity of the angles marked upon them. 1 b, an angular 

 geometrical elevation, alfo with the quantity of the angles 

 marked upon them, i c, one of the faces of the o6laedrou, 

 fiibdivided to -fliew the fuppofed arrangement of the primi- 

 tive moleculoe that form one lamina. The dark triangles rc- 

 prefcnt the fpaces they occupy refpcftively, and the diffe- 

 rence of fhading, the progrefs of diminution by a fubtrac- 

 tion of one range of moleculae on the edges of the faces, ac- 

 cording to the third law of decrement. The fuperior angle 

 A of this face is 70°, the two inferior angles E, E, ^^"^ each. 

 The letters on the three o-eometrical figures mark the fame 

 fituations on all of them, and are thofe employed by Hauy 

 to indicate the laws of decrement. 



As Ilauy's Jyfiem has been given in the Firft Volume of 

 the Fhilofophical Magazine, and his Jigns in the Second, 

 little more is required here but to give the fign to which the 



figures belong. 



I 

 Virjl haw of Decrement D. 



To this law belong fig. 2, 3, and 4. Fig. 2 ^, without the 

 dotted lines, would reprefent a plan of fig. 2. That portion of 

 fig. 2 rt, included beiween the dotted lines hh, hb, reprcfent» 

 a plan of fig. 3. The plan of fig. 4 is evident from the figure. 



Second l,mi< (f Dfcrc7nrnt 'E'. 

 Fig. 5 belongs to this law: fig. 2 a, the angles being taken 

 ofT at the doited lines, rcprcfcnts a plan of it. Fig. 6 is a 

 7 couibiuatioa 



