36 Dr. R. D. Thomson on the History [Jan. 



of Berzelius, that Del Rio was induced to alter his opinions 

 in reference to the composition of the Zimapan mineral, 

 out of respect to scientific authority is incorrect.* 



The Annates de Ciencias Naturales of Madrid, the medium 

 through which Del Rio communicated the change of his 

 views, although published by the Spanish botanist Cava- 

 nilles did not find their way among chemists in general. 



In the Journal de Mexico of the 11th September 1811, 

 Del Rio published a true statement of the case, an extract 

 of which was given in the Annates des Mines, (IV. Vol. for 

 1819,) transmitted by Alaman a Mexican mineralogist. 



Collet Des Cotils, who had heard of the discovery of Del 

 Rio, but unaware of the repetition of his experiments, 

 having been furnished with a specimen of the mineral, 

 analyzed it and published his results in 1805, a year after the 

 corrected analysis of the Mexican chemist had appeared. f 



Des Costils found the mineral to afford before the 

 blow-pipe upon charcoal, sometimes a slight smell of arsenic, 

 and to be speedily reduced. 



Heated with borax, it dissolved readily, and gave it a 

 light emerald-green colour. His method of analysis was as 

 follows : A portion of the mineral in powder, having been 

 digested with the assistance of a gentle heat, in dilute 

 nitric acid, the whole of it dissolved, forming a clear yellow 

 solution with a tinge of green, except a small quantity of a 

 reddish matter, which he found to be oxide of iron, contain- 

 ing a little silica and chromic acid. Into the solution, con- 

 centrated sulphuric acid being then poured, a white powder 

 fell, which was recognised as sulphate of lead. The filtered 

 liquor afforded no precipitate with ammonia. The excess 

 of ammonia was dissipated, and the acid thrown down by 

 nitrate of lead. The precipitate was yellow, resembling 

 the chromate of lead, and contained some sulphate of lead, 

 which was removed by taking up the chromate, by digestion 

 in nitric acid. In another experiment, the mineral was 

 dissolved in muriatic acid ; the excess of acid removed, and 

 the mineral acid thrown down by nitrate of silver. A fine 

 red precipitate was the consequence. With nitrate of mer- 

 cury, a yellowish compound was formed. 



* Kongl. Ventenskap. Acad Handlingar, 1831 — 1 

 t Anaales de Chimie, liii. 268. 



