ARSENIATED GOfPER, 255 



In your obfervations. Sir, you have raifed to 50^ 4' ahd 

 65° 8' the meafures which I had eftabliOied at 50'' and 65°; 

 meafures which you have fixed from the relations eftablidied 

 by you between the height of" one of the pyramids and the per- 

 pendiculars drawn from its bottom on the edges of its bafe, 

 which correfponds with the adjacent and unequally inclined, 

 pyramidal faces. Thefe meafures are fo near to mine, that I 

 have proved them again, and the inftrument is fo little capable 

 of marking this difference, that I do not make any difficulty in 

 adopting them. 



To conned the form of the fecond fpecies with the obtufe 

 odlahedron of the firfl, you afterwards fuppofe two fedions 

 made parallel to one of the moft inclined faces of the octahe- 

 dron, fo as to detach a very thin fegment, in whieh the centre 

 of thiso(5tahedron is to be contained; that is to fay, this octa- 

 hedron is to be confiderably increafed on all Its faces, with the 

 exception of one alone, taken on each pyramid, and in an 

 oppofite diredion in each of them. You fuppofe at the fame 

 time a decrement of a fingle row along the edges of the bafe, 

 but which ads only on two of the faces of the odahedron, 

 and that the fegment which refults on two of the three fides 

 inclined to each of the terminal facesj makes with them an an- 

 gle of 130|° and the third 115°, meafures which only differ 

 . 5° 30' in the angle of 130*^ 30' from thofe which I have given 

 for the cr) flal. 



The following is the anfwer didaled by the new examina- 

 tion which I have made of this fubfiance. 



Agreeably to what I have fiiid in my memoir on the arfeni- Objedlons to 

 ated coppers, the obtufe odahedron frequently Oiows fl'gli'^ J^f^JeJ^^'^""-'' 

 flreaks in its faces parallel to its edges, which indicates a la- 

 mellated texture in the diredion of thefe faces. The fradure 

 alfo indicates the fame texture, but its fradures are always 

 more or lefs irregular. I have never been able to obtain a 

 clear one. In the fecond fpecies, on the contrary, the lami- 

 r.ae are as eafily raifed from the hexahedral terminal faces, as 

 could have been done on a prifm of mica. Thefe terminal 

 j^aces are fometimes fireaked parallel to the edges of the fides 

 which are inclined to them, and thefe flreaks, which are conti- 

 nued ftrongly on the fides, never appear upon them except in 

 this diredion. This texture, very analogous to that of mica, 

 feems to me to be totally ditferent from that of the obtufe oc- 



tahej^ron of the firft fpecies. 



I have 



