and ShepardJ's Treatise on Mineralogy. 337 



double the number in Mg, (magnesia,) and in translating it 



into the chemical formula, MgS^ becomes, as above, Mg^'Si^, 

 (3 parts of magnesia and 2 of silica,) an expression less easily 

 printed, but more readily understood by the uninitiated; The 

 mineralogical formula would be read by the student as bisilicate 

 of magnesia ; but few would know that this bisilicate of magne- 

 sia consists of 3 atoms of magnesia and 2 of silica, as is read at 

 once from the chemical symbols. 



Thomson and Beudant are in general the authorities referred 

 to for the formulas, although Thomson's many errors are well 

 known. The late chemical treatise of Rammelsberg has con- 

 tributed somewhat to these tables, and so far they are unques- 

 tionable authority. 



The descriptions of the species are given with fullness, and 

 valuable remarks added respecting the uses of such as are em- 

 ployed in the arts. In no part of the work do Mr. Alger's labors 

 appear to better advantage than in the numerous interesting items 

 of information which he has brought into his descriptions, evin- 

 cing a minute and familiar acquaintance with minerals both in 

 the closet and field. The extensive enlargements of this part of 

 the work must have required great labor, and in general evince 

 discrimination and judgment. The long lists of American local- 

 ities render it a convenient work for the field, and the complete- 

 ness of the information given relative to the chemical, physical, 

 and economical characters of minerals, makes it as well a valua- 

 ble text-book for the lecture room. 



Nearly all that is new regarding American species has already 

 been noticed in the late numbers of this Journal. The following 

 are the characters of the Beaumontite of Dr. Jackson, cited from 

 pages 497, 498.* 



Beaumontite. 

 Crenated Hydro-silicate of Copper. C T. Jackson. 



" This mineral was obtained from the old carbonate of copper mine 

 of Chessy, France, in 1832. It was regarded as a hydrated silicate of 

 copper, but subsequent analysis proved it to be a crenated hydro-sili- 

 cate of copper. The analysis of a specimen which had effloresced 

 by exposure to the air yielded to Dr. Jackson : — Silica, 21-0 ; deutox- 



* See also this Journal, Vol. xxxvii, p. 398. 



