Roijal Society. 69 



nesia but no silica in solatlon. That the sokible silica was really 

 combined with magnesia was shown by boiling the insoluble mixture 

 with sal-ammoniac, which, dissolving the carbonate, left a large 

 amount of magnesia with the silica. This silicate was readily decom- 

 posed by hydrochloric aciJ, t'.ie greater part of the silica separating 

 in a pulverulent form. 



The third experiment was suggested by some observations on the 

 reactions of silicate of soda T.ith earthy carbonates. Kuhlniann has 

 remarked the power of carbonate of lime to abstract the silica from 

 a boiling solution of soluble glass*, and it is known that alumina 

 exerts a similar action. I have found that when artificial carbonate 

 of magnesia in excess is boiled with a solution of silicate of sodn, the 

 latter is completely decomposed with the formation of carbonate of 

 soda, and a silicate of magnesia which gelatinizes M'ith acids ; and I 

 have long since described this reaction in the evaporation of alkaline 

 mineral watersf . This mutual decomposition of carbonate of mag- 

 nesia and silicate of soda, conjoined with the power of carbonate of 

 soda to dissolve silica, leads to a curiou? result. If we boil for some^ 

 hours a mixture of ignited silica, obtained from the decomposition of 

 a sihcate by an acid (and consequently readily soluble in an alkaline 

 carbonate), with a small portion of carbonate of soda and an excess of 

 hydrocarbonate of magnesia, we obtain a dense powder which contains 

 ail the silica united with magncjia, and may be boiled with carbonate 

 of soda and sal-ammoniac without decomposition. It is obvious from 

 the above experiments that similar results may be obtained with 

 quartz, although the process is much slower ; it would doubtless be 

 accelerated under pressure at a somewhat elevated temperature, 

 which would enhance the solvent power of the alkaline carbonate. 



Silicates of potash and soda arc everywhere jnxsent in sedimentary 

 rocks, where decomposing felspathic materials are seldom wanting, 

 and these salts in the presence of a mixture of quartz and earthy 

 carbonates, aided by a gentle heat, will serve to effect a union of the 

 quartz witli the earthy bases, eliminating carbonic acid. A small 

 amount of alkali may "thus, like a leaven, continue its operation in- 

 definitely and change the character of a great mass of sedimentary 

 rock. Such a process is not only a possible but a ijecessary result 

 under the circuniotanccs supposed, and we cannot, I think, dnnbt 

 that alkaline silicates play a very important part in the metamor])hism 

 of sedimentary rocks, which arc composed for the most part of earthy 

 carbonates, with siliceous, aluminous, and felspathic materials^. 



Tiie direct action between the carbonates and silica nuist neces- 

 sarily be limited by their mutual insolubility, and by the j)rotecting 

 influence of the first-formed portions of earthy silicate ; but with the 

 solvent action of a small ])ortion of alkali which is changed from 



* Coiiiptcs Rcmlus dc I'Acail. dcs Sciences, Dec. Srd nml Dec. lOtli, 1855, 

 where will he. found many important observations on the alkaline silicates. 



t Reports of the Ge(d. Survey of Canada, 18.'>l-53-51. 



t It is well known that small portions of alkalies arc seldom or never wantini; 

 in the earthy silicates, buch as serpcMtiiic, talc, pyroxene, asbestus, cpidote, ido- 

 erase, and even beryl and corundiun. See tlic memoir of Kuhlmann already cited. 



