69 PHYSIOGRAPHY. CLASS n, 



distinct faces of cleavage. Colour white, some- 

 times reddish or greyish. Streak white. Trans- 

 parent ... translucent. 



Brittle. Hardness = 5-0. Sp. Gr. = 2-694 of a 

 crystallised variety from Tolfa. 



Compound Varieties. Massive : composition small 

 granular, often impalpable; fracture uneven, flat 

 conchoidal, splintery, sometimes earthy. In the 

 interior of the massive varieties, there are cavities 

 lined with crystals. 



OBSERVATIONS. 



1. This species consists, according to CORDIER, who 

 analysed a variety from Tolfa, of 



Sulphuric Acid 35-495. 



Alumina 39-654. 



Potash 10-021. 



Water and loss 14-350 ; 



with a trace of oxide of iron. Upon charcoal before the 

 blowpipe it does not melt by itself, nor with soda, but is 

 fusible into a colourless globule with borax. Reduced to 

 powder, it is soluble in sulphuric acid. 



2. Rhombohedral Alum-haloide is found at Tolfa near 

 Civita Vecchia, in the vicinity of Rome ; also in Tuscany, 

 in the kingdom of Naples, and in the county of Beregh in 

 Hungary, also in the neighbourhood of several active vol- 

 canoes. It seems to form beds in these places. According 

 to PRZYSTANOWSKY, the beds of rhombohedral Alum-ha- 

 loide in Italy occur in Quartz, which is here and there 

 mixed with clay. According to others it is found in veins 

 and geodes of a particular kind of rock, which has been 

 called the Alum-rock, and which probably consists of compact 

 varieties of the same species. 



3. It is used in the manufacture of alum ; and the su 



