Geological Collections. 191 



seem to be at right angles to one another. Its colour is tin white, almost like 

 antimony, but a little more yellow ; it has much metallic lustre, is brittle, 

 and of the hardness of fluor spar: spec. gr. = 8.159. It is mixed with 

 small proportions of tellurium-silver, and before the blowpipe, on charcoal, 

 fuses to a small button, which gradually diminishes in size, so as ultimately 

 to exhibit a small globule of silver, surrounded by a ring of metallic hue, 

 which seems to be formed by the volatilized and subsequently precipitated 

 tellurium-lead. If the flame is directed upon it, it is completely volatilized, 

 the flame becoming at the same time of a blue colour. It fuses also in a 

 retort, and forms a small quantity of white sublimate, which, under the 

 action of strong heat, contracts into small globules. If ignited in an open 

 tube, it fuses, and becomes surrounded by a ring of white drops ; and, at the 

 lower portion of the tube, a very dense white sublimate is deposited, which, 

 before the flame of the blowpipe, contracts into small drops. 



As one analysis only could be made of the mineral, M. Rose refrains 

 from giving any decided opinion on its composition at present ; he is, 

 however, inclined to consider it as a compound of 1.28 of silver, 60.35 of 

 lead, and 38.37 of tellurium Royal Inst. Journ. Feb. 1831. 



Voltaic Test of the State of Metals in Mineral Substances It is well known 



that Dr Wollaston devised a beautiful little arrangement to ascertain the 

 conducting power of certain crystals having metallic characters, and which 

 ultimately proved to be titanium. If a plate of copper be in contact with a 

 plate of zinc, and part of both plates be immersed in a dilute acid, the 

 copper, by its electric condition, decomposes water, and becomes covered 

 with bubbles of hydrogen. If a piece of paper, or a card, be interposed 

 where the two metals were in contact, the copper loses this power altogether, 

 and no bubbles appear on it ; but if a small hole be made in the paper or 

 card, and a little piece of metallic matter put there, so as to touch at once 

 both the zinc and copper, then the latter has its full power restored. 



M. Macaire Prinsep has applied this test more generally ; and he found, 

 in the first place, that a metal was necessary to restore the eflfect — lead, 

 bismuth, tin, &c. reproduced the bubbles ; but sulphuret of arsenic, rutilite 

 or oxide of titanium, gray cobalt ore, and the sulphurets of antimony, iron, 

 tin, or lead, produced no eflfect. Portions of meteoric stone from Aigle 

 and Barbotan, by producing bubbles, shewed that they contained uncombined 

 metal ; and the method seemed competent to indicate, in all cases, whether 

 the metals used were free, or in a combined condition. 



As lead gave bubbles, but the sulphuret of lead none, experiments were 

 made with lead, to which sulphur, in increasing proportions, had been added : 

 — 1-lOOth, l-50th, l-32d, l-16th, and l-12th of sulphur, did not take away 

 the property from lead ; but when l-8th of sulphur was used, no bubbles 

 appeared upon the copper. Then ascertaining the proportions in the definite 

 sulphuret of lead, he found them to be exactly those which caused the 

 evolution of bubbles to cease (86 lead and 14 sulphur.) The same effect 

 occurred with the sulphuret of tin ; and hence it was concluded, that chemical 

 combination in determinate proportions was necessary to prevent this electric 

 decomposition, and that mixtures had no influence on the phenomena. 



These results may be important to the mineralogist ; and M. Macaire 

 Prinsep, in illustration, concludes, that the gray cobalt ore of Lunaberg, 

 which is composed of cobalt, arsenic, and sulphur, contains only sulphurets 

 of the metals ; that, on the contrary, the metals of aerolites, although some- 

 times found associated with sulphur, and always with silica, exist neither as 

 sulphurets nor silicates, but in their metallic condition. — Bib. Univ. 1830, 

 p. 146. 



