DR T. THOMSON on Asbestus, Chlorite, and Talc. 365 



It is obvious at first sight that this mineral is quite distinct 

 from amianthus, asbestus, and rock-wood ; for it is almost desti- 

 tute of magnesia, which in all of them constitutes the most 

 abundant of the bases. 



It is not easy to decide whether the water which exists in 

 such abundance in mountain-leather, be chemically combined or 

 not ; for it imbibes water as readily as a sponge. It was kept 

 in a dry room, till it ceased to lose any more weight, and in this 

 state was subjected to analysis. When exposed to redness, it 

 altered its appearance very much, and no longer bore its former 

 resemblance to leather. I am disposed, from this, to admit wa- 

 ter as a chemical constituent of this mineral. 



The constituents of mountain-leather are equivalent to 



80 atoms silica, 

 1 3 atoms alumina, 



9 atoms lime, 



2 atoms protoxide of iron, 



1^ atom magnesia, 

 60 atoms water. 



The four bases taken together amount to 25-^ atoms, while 

 the silica amounts to 80 atoms ; therefore the alumina and lime 

 must be combined each with three atoms of silica, while the 

 protoxide of iron and magnesia must be in the state of quater- 

 silicates. 



Farther, the atoms of quatersilicates being 3^, while those of 

 tersilicates are 21, it is obvious that there exist in the mineral 

 6 times as many atoms of tersilicates as of quatersilicates. For 

 3.5 x 6 = 21. 



The ratio of 1^ to 2 is the same as that of 3 to 4 ; and that 

 of 9 to 13 approaches very nearly to the same. Hence the 



z z 2 



