and Partial Solution of Bocks, Minerals, 6fc. 167 



lies are in the form of carbonates ; the iron, in the case of 

 hornblende, epidote, &c., passing from the state of carbonate 

 to that of peroxide during the evaporation, collects in brown 

 flocculi, along with the silica and alumina, at the bottom of 

 the capsule. Thus, 40 grains of hornblende, digested for 

 forty-eight hours in COj water at 60°, with repeated agitation, 

 yielded silica 0-08, oxide of iron 0-05, lime 0-13, magnesia 

 0095, manganese a distinct trace. 



(4.) Most of the substances above enumerated, when finely 

 powdered in an agate mortar, and moistened with pure water 

 in a platinum capsule, give decided alkaline reaction with test- 

 paper properly prepared. Among the materials presenting 

 this effect most strongly, are serpentine, chlorite, tremolite, 

 asbestus, mica, hornblende, the felspars, and glass. The 

 effect is especially striking with powdered glass. But it is 

 important to note, that this reaction is more immediate and 

 stronger with the magnesian and calcareo-magnesian silicates,, 

 than with the felspars and most other alkaline minerals. 



In making this, and all the other experiments embraced in 

 the present inquiries, it is, of course, necessary that the speci- 

 men should be free from any adhering carbonate of magnesia 

 or lime, either of which would give rise to an alkaline reaction, 

 and the former in quite a marked degree. It is, moreover, 

 necessary to avoid using either a wedgewood or glass mortar, 

 as the abraded matter would, in such a case, give to the car- 

 bonated water quite a discernible amount of alkali. 



(.5.) The comparative readiness with which the magnesian 

 and calcareo-magnesian silicates yield to the decomposing 

 and dissolving action of carbonated and even simple water, 

 it is, we believe, a fact no less important than it is true. It 

 explains the rapid decomposition of the rocks composed 

 mainly of hornblende, epidote, chlorite, &c., without calling 

 in the agency of an alkali ; and it accounts for the fact, that 

 rocks of this kind are often much more rapidly decomposed 

 by meteoric actions than even the felspars themselves. It 

 enables us, moreover, to trace the simple process by which 

 plants are furnished with the lime and magnesia they require, 

 from soils containing these silicates, without our having re- 



