4* On Argantal Mercury, 



The fmall lamellaefonr. leaves of argental mercury are for 

 tlie mod part bent, and follqw ihe undulations of the rock to 

 which they are applied. Their furface is. generally fnioolh 

 and poliil'cd, hut much lefs than that of iu cryftals. This 

 metal is eaiilv icratched by a piece of (liarp-pointed fteel. By 

 fcfiiping it loie'i ahnoft all its fplcndour, and becomes dull. 

 When rubbed on copper. it leaves a white metallic trace. It 

 is brittle and eafily broken : its confulence approaches to that 

 of martial pyrites. Its frafture is conchoid, and exhibits no 

 appearance of laniin:s. The fragments of it are iiulcter- 

 mined, with very obtufe edges. 



Its fpecific gravity, determined from a mean of feveral ex^ 

 periments, is 14*1 192; argcnfal mercury therefore, next, to 

 platina and gold, is the h-eavieii: of bodies. When this mi- 

 iioral is heated at the blow-pipe, the. mercury becomes vola- 

 tilized, and a fma'l button ot lilver may be eafily obtained. 



The vari<eties of the regular forms are, i(t, Theperteft 

 rhomboidal dodccacdron (fig. i. Plate III), The incidence 

 of the two cx)nti2uous faces is 120 . The cryftals not being 

 fufccptiblc of any mechanical divifion, it is not poOible to 

 know precilelv whether tliis folid be ,the primitive form of 

 argental mcrcurv, as is probable, and as we (hall I'uppofe it 

 to be, in order to have the expreffion of the laws of decrc-r 

 ment and the value of the angles. This fuppolition can pro- 

 duce no error, becaufe the reiuUs of the calculation may be 

 eafilv transferred, fo as to apply them to the octaedron, the 

 tetraedron, or the cube, which are the only other forms 

 poffible. 



2d, The dodccacdron truncated on the fix folid angles 

 compofcd of four planes. The fix new faces are produced ia 

 virtue of a decrement by one row : they belong to the cube, 

 and make with the faces of the primitive forn) angles of 135°. 

 According to the ingenious method of C. Haiiy, the abridged 

 exprclfion of the laws of decrement which produce this 



form is P E, 



3d, The fame as the preceding, the place of each ridge of 

 which is fupplicd by a facet making an angle of 150 with 

 the adjacent primitive face : thefe new facets take place by the 

 fubtradi'in of a row of molcculje on all the edges. Its exr 



I : I 



preffion is P B E. 



I 



' 4th, The dodccacdron truncated on all the ridges and alf 



the lolid angles, and having new facets on the edges of the 



Iruncatures which take place on the edges, and the foli4 



angles compofcd of four planes. This form, which had not 



been 



