256 Scientific Proceedings, Royal Dublin Society. 



lustre dims in a couple of days if the crystals be left exposed to the 

 moisture and carbonic acid of the air, ultimately becoming opaque 

 and falliug to pieces. Examined between crossed nicols, they are 

 found to be optically inactive, a conformation of their true cubical 

 character. 



Their formation is by sublimation, a volatilization and subse- 

 quent arrangement of the molecules occurring at the high temper- 

 ature of the flame. They are often, indeed, transported an 

 appreciable distance from the point of greatest heat, building up 

 in the manner of hoar frost, on the cooler surfaces. This frost of 

 calcium oxide is in various branching forms, with parallelism 

 maintained between the faces of the cubes on any one branch, the 

 cubes being really continued one into the other, in a manner re- 

 calling octahedral strings of magnetite. 



The cubes are hardly greater than one-fifth of a millimetre on 

 the edge. Under the microscope their brilliancy is very striking. 



So far as I can ascertain, ciwstallized calcium oxide has not 

 before been directly prepared from the amorphous substance. 

 Briigelmann has obtained it by heating the nitrate in a porcelain 

 flask. 1 



Magnesium Oxide. — Pure magnesia, which has been caused to 

 cohere by wetting it with water containing a little sugar, and then 

 drying in small pieces, when exposed to the oxyhydrogen flame in 

 a similar manner to that described in the case of calcium oxide, 

 also develops small crystals. In this case the heating must be 

 more prolonged, as the formation of ciwstals is less marked. 



The microscope shows these crystals to be cubical in form, clear 

 or translucent, and colourless, with high lustre. They are very 

 minute, and are formed on the edge of the enamel of melted 

 magnesia. 



Regular cubes of the oxide have been previously obtained by 

 igniting the amorphous substance in a current of hydrochloric acid, a 

 and in this way, too, Deville prepared the mineral form of the oxide, 

 periclase, which contains some iron. 



1 "Treatise on. Chemistry," Roscoe and Schorlernnier, vol. n., pt. i., p. 190. 

 Watt's Diet., new ed., vol. i., "calcium oxide." 



2 Roscoe and Schorlemmer torn, cit., vol. ir., pt. i., p. 240. 



