S54 DR T. THOMSON on Asbestus, Chlorite, and Talc. 



subjected a good many varieties of asbestus to a chemical ana- 

 lysis, shewed that they were not composed of a particular earth, 

 but that they all contained magnesia, and therefore arranged 

 them under Magnesian Earth *. This arrangement was followed 

 by WERNER, and has in consequence passed into almost all the 

 modern systems of mineralogy. 



There are usually reckoned five varieties or subspecies of As- 

 bestus ; namely, Amianthus, Common Asbestus, Rock-wood, 

 Mountain Leather, and Mountain Cork. 



1. Amianthus. 



As a specimen of amianthus, I chose a variety from Sardinia, 

 very like the well known Corsican amianthus, which is suffi- 

 ciently pure, and so abundant, that DOLOMIEU made use of it 

 for packing his minerals. It is composed of very fine threads of 

 considerable length, and easily teased from each other. Its lustre 

 is silky, it has a greenish-white colour, is very soft, and has a 

 specific gravity of 1.551. Its constituents, analyzed in the usual 

 way for treating magnesian minerals, were found to be as follows : 



Silica, 55.908 or 35 atoms, 



Magnesia, .... 27.068 . .13^ atoms, 



Lime, 14.632 . . 5 atoms, 



Protoxide of iron, . 6.528 . . 2 atoms, 



Alumina, .... 1.820 . . 1 atom. 



105.956 

 We see from this analysis, that amianthus contains four 



* Opus. iv. 160. 



