412 Prof. B. Sillimaiij */i\, on Gibbsite and Allophane. 



sis of Gibbsite, from a desire to satisfy my own doubts regarding 

 it, and in the belief that it was quite probable that phosphate of 

 alumina might have been regarded as alumina by those who had 

 analyzed the mineral before M. Hermann, from the striking 

 similarity of the two substances in nearly all their reactions. It 

 will be seen in the sequel, however, that my results go to rees- 

 tablish the accuracy of Dr. Torrey's analysis and the ancient for- 

 mula of this species. 



i was bountifully supplied with excellent specimens of Gibbs- 

 ite from the original locality in Richmond, Massachusetts, by 

 the kindness of Dr. Stephen Reed, a gentleman well known to 

 American mineralogists. Among the specimens sent to me were 

 all the varieties of form which stalactitie and encrusting mine- 

 rals ever assume, and many specimens were of remarkable size 

 and perfection. Some specimens of the Gibbsite were covered 

 with a pearly white lamellar mineral, encrusting and intimately 

 associated with it, and which closely resembled allophane, which 

 mineral, analysis proves it to be. 



Physical Description. — Gibbsite occurs in stalactites often sev- 

 eral inches long, in mamillary masses, and incrustations covering 

 broad surfaces of iron ore and ferruginous rocks. The stalactites 



have a radiating subfibrous structure, and always enclose particles 

 of iron ore as a nucleus. The crusts are generally the most free 

 from foreign matter. No crystalline structure could be detected 

 in any of the specimens. Hardness 3i> — 3 75. Density 2*389. 

 Lustre faint subvitreous. Streak white. Color white and gray- 

 ish white, sometimes reddish and greenish. Yields a strong ar- 

 gillaceous odor when moistened or breathed on. Texture tough 

 and adheres strongly to the polished surface of the agate pestle 

 and mortar. 



Pyrognostic characters. — In the close tube gives off water 

 abundantly which is neutral, but has a decided empyreumatic 

 odor. On charcoal it decrepitates strongly from the escape of wa- 

 ter, and finally becomes opake ; phosphoresces ; moistened with 

 nitrate of cobalt-solution, it gives when heated upon charcoal a 

 bright blue slag like pure alumina. It dissolves to a transparent 

 bead ill salt of phosphorus acid, and in borax. Treated with the 

 iron wire for phosphoric acid not the least evidence could be ob- 

 tained, according to the method advised by Berzelius. By em- 

 ploying the method advised by Vauquelin and Thenard for de- 

 lecting phosphoric acid in small quantity by means of potassium 

 in a deep tube, a decided but not strong odor of phosphuretted 

 hydrogen was obtained. 



Chemical characters. — The finely pulverized mineral dissolves 

 at once in strong potash or soda ley, leaving only a few Qocs of 

 peroxyd of iron. In boiling concentrated chlorohydric acid it is 

 only in part soluble, unless after a very long time and with re- 



