On a native Arseniate of Lead. Qg 



the Brazilian topaz : this, in the greater nuinher of speci- 

 mens, passes into a delicate Isabella- colour : whilst, in other 

 cases, we have the honey -yellow mingled with brown hues 

 ot'difTcrent intensities : so tliat we meet with crystals re- 

 sembling dark brown sugar-candy, or common resin. 



Some of the crystals are beautifully transparent, whilst 

 others possess this quality in part only, at their extremities, 

 or in inferior degrees throughout their whole lengths. 



The external lustre, in some specimens, is vitreous; in 

 others, resinous : but in some instances their surface is 

 partially covered by tender and delicate filaments of a silky 

 lustre. These filaments are sometimes found in a separate 

 state loosely adhering to quartz ; and they form a variety of 

 this fossil. 



The crystals vary as to hardne^js. The angular frag- 

 ments of the most transparent are sufficiently hard to scratch 



^^f^\ ...» 



This mineral is easily reduced to powder, which has the 



appearance of pounded resin ; it contracts a yellower tint 



by long exposure to the air. 



The specific gravity of the purest crystals, taken at the 



temperature of 50° Fahrenheit, was 6*4 1. 



in. 



A fragment of crysta), exposed to the flame of the blow- 

 pipe in a gold spoon, melted into a brownish-vellow mass, 

 which on cooling did not assume any angular figure. It 

 remained in a state of ignition apparently unaliered; but 

 when a piece of it was exposed to the flame on charcoal, a 

 rapid decomposition took place, arsenical vapours were ex- 

 tricated, and globules of a metal, possessing the common 

 properties of lead, were left behind. 



This mineral, in a state of fine powder, is soluble in 

 nitric acid, even without the aid of heat. Care, however, 

 must be taken, that it does not concrete into lumps. The 

 vessel therefore which contains it must be frequently shaken, 

 and the nitrate of lead produced must be, from time to 

 lime, dissolved in water, and poured oft^ froni the residuum. 

 The process of solution is, however, accelerated bv a di- 

 gesting heat. Some silica remains, which, as the (juantity 

 of it is variable according to circumstances, appears not to 

 be an essential inoredicnt of this fossil. 



The nitric solution is colourless; its transparency is not 

 disturbed by nitrate of barytes. Nitrate of silver renders it 

 turbid, and a small quantity of white curdly matter is de- 

 posited. Sulphuric acid and the liquid sulphates produce 



copious 



