Alexandre BRONGNIART (1770-1847), mineralogist. Autograph letter signed to the English botanist Philip Barker Webb (1793-1854). 2 pp. in-8. 7 January 1839. Address on the back. Slight wetness but not serious. Analysis of rocks sent by Webb. "I send you back, sir, your named rocks. They presented me with more difficulties than I anticipated at first sight, and to make my determination clearer and more reasoned, I have added to the name the characters that these rocks represent. You can use them as you wish. I have taken the liberty of keeping a sample that is completely duplicate in every respect of this Leucostous n°8. It's a specimen that I don't have of a new modification, at least that's how I see it". He was able to see Mr Berthollet but did not have information about the vacancy in Toulouse. He will talk to Moquin-Tandon about it.