I 



Hardman — On the Carboniferous Dolomites of Ireland. 711 

 £liiclc Magnesian Limestone. From Stadtbergen.*' 



I. A^^ALYSIS. 



Carbonate of lime, 84-57 



,, magnesia, 11-54 



,, iron, 1-15 



Silica and carbon, 1-36 



98-62 



II. Amount of Co^fsxiTUEXTs dissolved in 24 hours, from 6660 grains. 



Grains. 



Carbonate of lime, 4-29 



Carbonate of magnesia, no trace. 



There appears to be, for the quantity taken, and the time occupied, 

 more carbonate of lime dissolved than occurred in any of my experi- 

 ments ; however the powdering of the rock might give rise to this. 

 J3ut this important fact still remains, that, when treated with a weak 

 solution of carbonic acid, limestone yields more carbonate of lime pro- 

 portionally than carbonate of magnesia. 



Perhaps it is well to notice here the curious difference that pres- 

 sure makes, not only in the solvent power of carbonic acid, which 

 is increased, but in its relative effects on the two carbonates. It 

 appears certain that, under the ordinary atmospheric pressure, and in 

 such propoi'tions as it occurs in most surface or even underground 

 waters, it will chiefly attack the lime, while under a high pressure, 

 and in large quantity it seems to confine its attentions, I may say, 

 strictly to the magnesia. Advantage has actually been taken of this 

 property to procure salts of magnesia, such as the sulphate, from 

 dolomite ; the process consisting in submitting the rock, finely 

 ground, with water, to the action of carbonic acid, under a pressure of 

 about four atmospheres. It is then found that nearly all the carbonate 

 of magnesia is removed, without admixture of carbonate of lime.f 



This is worth noting, as it may serve to account for the large 

 quantities of carbonate of magnesia which are occasionally, hut not 

 often, found in spring waters ; and may also explain the production of 

 deej)-seated dolomites by infiltration of magnesian water. 



As it is, however, with dolomites formed under circumstances not 

 taking in the element of pressure we shall have to deal at present, 

 I sliall not enter more fully into the above matter just now; but pro- 

 •ceed to descnbe the experiments I have referred to. 



It appeared to me that if it could be proved by some process as 



* Bischof, op. ei(., vol. iii., p. 195. 



t Dingl. Polyt. J., ccLx., 467. — Abs. Jour. Chem. Soc, vol. xii., p. 96. 



