Mr* Pentland's Observations on the Peruvian Andes. 115 



and realgar ; that is to say, that the gray mineral will be ex- 

 pressed by the formula Zn Se 4 + HgS. The red mineral by 

 the formula Zn Se 4 + H#S 2 . 

 Mexico, December 1, 1827. (Signed) A. Del Rio. 



On one occasion I distilled the mineral alone, and I 

 poured spirit of wine into the receiver, when I observed at the 

 bottom a drop of yellow oil, which in time tinged the alcohol 

 of a beautiful yellow : on the addition of water, the colour dis- 

 appeared without any precipitate being thrown down. I pre- 

 sume that this was the same substance noticed by Berzelius, 

 as being formed on the admixture of selenic acid and anhy- 

 drous muriatic acid with selenium, in which case both these 

 acids must exist in the mineral. I detected the muriatic by 

 means of nitrate of silver; but no sensible precipitate of sele- 

 niate of silver was obtained by the addition of cold water to 

 a boiling nitric solution, perhaps because the quantity was too 

 small. 



XVIII. Observations on the Peruvian Andes, in reply to a 

 Paper by M. Coquebert de Montbret, in the Annales des 

 Sciences Naturelles. By J. B. Pentland, Esq, 



To the Editors of the Philosophical Magazine and Annals. 

 Gentlemen, 

 r iPHE last Number of the Annales des Sciences Naturelles, 

 -*■ (vol. xiii. p. 420) contains a paper by Mons. Coquebert de 

 Montbret, entitled " Note sur quelques Montagnes du Haul 

 Perou" purporting to be founded on a memorandum of some 

 of my measurements of the Peruvian Andes, which I had 

 communicated to that gentleman in February last. 



The memorandum in question was drawn up, in the course 

 of conversation, for Mons. de Montbret's private information ; 

 and I distinctly stated to him at the time that the results 

 ought merely to be considered as approximative ; since I had 

 not the detailed notes of my observations at hand, and the 

 calculations on which they were founded, required a careful 

 revisal, before they were submitted to the public, — having been 

 made in the midst of the fatigues and annoyances attendant 

 on a tedious journey. I have therefore seen with regret, 

 that a part of my observations has been rendered public in 

 the unconnected and inaccurate manner in which they are 

 brought forward in the paper in question : and I shall feel 

 sincerely obliged by your giving an early insertion to this 



Q 2 letter, 



