710 Proceedings of the Royal Irhh Academy. 



tigate the comparative solubility of carhonate of lime and carhonate of 

 magnesia in carhonated water. 



The means whicli they employed were — 1st, what they designate 

 as the method -with the tache, consisting in digesting for a few minutes 

 a small quantity of the mineral, finely poivdered, on a filter with car- 

 bonated water, and then collecting the filtrate and examining it for 

 lime and magnesia. 2nd. By agitating briskly for some time, in a large 

 glass bottle containing carbonated water, a quantity of the mineral, in 

 this case oim finely poicdered. The water was then evaporated, and the 

 residue examined. In both these cases, magnesian limestone so treated 

 yielded a larger quantity of carbonate of magnesia than of carbonatc- 

 of lime, proportional to their relatiye amounts in the rock ; and the 

 Professors Eogers infer that in nature this process would result in the 

 limestone hecoming less magnesian, instead of approacliing to a dolomitey 

 as is generally maintained. 



jS^ow I wish to point out that the process sketched above cannot 

 by any means be held to represent that which takes place in nature. 

 The veiy act of powdering the dolomitic limestone has destroyed any 

 value the experiment might otherwise have had. We do not find rocks 

 in situ thus prepared for the invading action of carbonic acid ; and we 

 know that dolomites entirely, and magnesian limestones to a great 

 extent, resist the action of much stronger acids than a merely car- 

 bonated solution, so long as they remain solid ; but once they ai'e 

 powdered up, they are readily dissolved with evolution of carbonic 

 acid. In effect, this fact is made use of in testing rocks in the field ; 

 dolomitic limestone being scarcely affected at all by moderately dilute 

 hydrochloric acid, and can therefore be readily distinguished from 

 ordinary limestone. 



I do not know if the amount of magnesium carbonate obtained by 

 the above method was quantitatively determined by Professors Rogers, 

 as I have not been able to consult their detailed paper in the Ameri- 

 can Journal of Science ; but it is curious that Bischof obtained just 

 the contrary result to theirs, in the two experiments I have already 

 referred to, although his method of proceeding is essentially the same. 

 His results agree very well with my own. 



Biscliofs Experiments. The composition of the limestone being as- 

 certained, a portion was powdered finely, and placed in water for 24 

 hours. The water was then examined, and proved in the cases tried 

 to contain either no trace, or a very small one, of magnesia. I shall 

 copy one of these for example, as it will be useful to compare with my 

 results. 



