TOL. LXXXIX.] PHILOSOPHICAL TRANSACTIONS. 553 



The calcareous spar however dissolved more slowly where there was argillaceous earth, 

 as it became coated with it; but this incrustation was occasionally removed, and, in 

 all the experiments, the spar was left in the solution till it suffered no further diminu- 

 tion. For this purpose it was necessary to keep them slightly warm for some days, 

 during which time the phials were generally closed, to prevent any escape of the acid. 



The first experiment in the following table was made on known quantities of 

 magnesia and calcareous earth, to try the accuracy of the process. For this pur- 

 pose also, the 2d was repeated on a piece of limestone, previously powdered, to 

 render every part of it of the same quality. The first column shows the quantity 

 of calcareous spar which might have been dissolved by the acid required to take up 

 the magnesia. The 2d shows the corresponding quantities of magnesia in 25 gr. of 

 each substance. The 3d expresses the quantity of lime. This was inferred by 

 subtracting the weight of the magnesia, and of the iron and clay, from 13.2 gr., 

 the weight of the whole quantity of earth in 25 gr. of limestone. This is pro- 

 bably not very incorrect, as, in 2 specimens which differed most in the proportion 

 of magnesia and lime, the weight of the 2 earths was nearly the same. 



A piece of Dolomite, from Rome, was wrapped in a thin leaf of platina, that no part 

 of it might be lost, and, being then exposed to a strongheat, left of earth 52.Q per cent. 



Dolomite from Mount Vesuvius 52.8 



Breedon limestone 52.4 



Calcareous spar left of lime 55.8 



In 3 of the experiments also the calcareous earth was precipitated by mineral al- 

 kali ; and the quantity of it being tried by that of the marine acid required to 

 dissolve it, it corresponded very nearly with that set down. A quantity of marine 

 acid which would dissolve 15 gr. of calcareous spar, would also dissolve 5.5 of cal- 

 cined magnesia, and 2.5 gr. of spar; so that, 12.5 gr. of spar required the same 

 quantity of acid as 5.5 gr. of magnesia. The magnesia used was very pure, and 

 made red-hot immediately before it was weighed. 



Substances examined. 



Mixture of 5.5 gr. of magnesia and 14 gr. of calcareous spar 



25 gr. of Breedon limestone, previously powdered , 



25 gr. from part of the same powder , 



25 gr. of Dolomite from Rome 



Dolomite from Iona 



Vesuvian Dolomite 



A 2d experiment, from part of the same Vesuvian Dolomite . . 

 25 gr. of magnesian limestone fromWansworth, nearDoncaster 



Thorpe arch 



Matlock 



■ York Minster 



■ Worksop 



Sherborn 



— — Westminster-hall 



VOL. XVIII. 4 B 



I 111 



