264 



THE GEOLOGIST. 



mer. It consisted in dividing the mineral products of the 

 empire into four great general divisions, having in the centre 

 those obtained from the rivers, and those procured from the 

 principal chains of mountains, as the boundaries of either 

 side. The cabinets were so arranged as to form a kind of 

 section of the various geological formations. The upper 

 portions of the cabinets are filled with the rocks and mine- 

 rals from the heights or mountainous districts, whilst the 

 lower divisions contain the specimens taken from the val- 

 leys. This arrangement has been found greatly to assist the 

 memory, and to afford numerous points of comparison to 

 those who study the constitution of the mountain chains. 

 The Professor concluded by some speculations on the chan- 

 ges which gradually take place in the metamorphic rocks, 

 and which he considered might all be reduced to processes 

 of oxidation or reduction. 



On some new Oxides of certain of the Metals of the Magnesian 

 Family, by Br. Lyon Play fair. 



Dr. Playfair first adverted to our defective knowledge of 

 the history of the magnesian oxides. Chemists were more 

 intimately acquainted with this family than with any other; 

 but still the actual amount of their knowledge was very 

 scanty. Manganese, for example, possesses six degrees of 

 oxidation ; whilst magnesium, the type of the family, pos- 

 sesses only one. Iron and chromiuai possess sesquioxides, 

 but copper and zinc do not ; yet a complete identity in the 

 structure of their molecules has been pointed out, affording 

 new proofs of chemical analogy. Dr. Playfair showed a 

 diagram representing all the magnesian oxides now known, 

 in contradistinction to those which analogy leads us to ex- 

 pect. In this tabular statement, he denied the existence 

 of peroxides of copper, zinc, and calcium, although it is well 



