552 PHILOSOPHICAL TRANSACTIONS. [ANNO 17QQ* 



destructive to vegetables, when used in large quantities. I therefore procured some 

 pieces of it, and they were discoverd to contain nearly the same proportion of mag- 

 nesia as that before described. In this quarry, the stone is frequently crystallized 

 in a rhomboidal form ; and petrified shells, not calcareous, but similar in compo- 

 sition to the stone itself, are sometimes, but very rarely, found in it. This sub- 

 stance seems to be common in Northumberland. In the 3d vol. of the Annals of 

 Agriculture, Dr. Fenwick, of Newcastle, observes, that the farmers of that country 

 divide limes into hot and mild. The former of these is no doubt magnesian, as it 

 has similar effects on the soil ; and he remarks that it is not so easily dissolved in 

 acids as the latter. At Matlock, in Derbyshire, the 2 kinds are contiguous to 

 each other; the rocks on the side of the river where the houses are built being 

 magnesian, and on the other, calcareous. The magnesian rock appears also to be 

 incumbent on a calcareous stratum ; for, in descending a cave formed in this rock, 

 a distinct vein of common limestone may be observed, which contains no magnesia. 

 The latter stratum is very full of shells; but though there are some also in the 

 magnesian rock, yet they are very rare. In the following tables, containing the 

 analysis of various specimens, some other places are mentioned where this sub- 

 stance is found, but of which I received no further information. 



After it was known that the magnesian marble and limestone consisted of the 2 

 earths, their proportion was attempted to be discovered, by trying how much 

 gypsum and Epsom salt could be obtained, by means of vitriolic acid, from a 

 certain weight of each specimen. When the superfluous vitriolic acid had been 

 evaporated by heat, the Epsom salt was separated from the gvpsum by water. The 

 result of these trials is expressed in the following table. 



Dry gypsum. Dry Epsom salt. 



5 gr. of limestone from Breedon gave 3.9 3.15 



Matlock 3.95 2.9 



Worksop 3.8 3.0 



York 3.8 3.1 



3 gr. of calcareous spar and 1 gr. of calcined magnesia gave 3.9 2. 7 



As the preceding method of estimating the quantities of magnesia and calcareous 

 earth is liable to considerable error, I afterwards examined them in the following 

 manner, which seems capable of great exactness. Twenty-five grains of each sub- 

 stance were dissolved by marine acid, in a cup of platina, and after the solution 

 was evaporated to dryness, it was made red-hot for a few minutes. The mass re- 

 maining in the cup, which consisted of muriated lime, and of the magnesia freed 

 from the acid, was washed out with water, and poured into a phial. There was 

 then added to it a known quantity of diluted marine acid, somewhat more than was 

 sufficient to re-dissolve the magnesia, and, after the solution, a certain weight of 

 calcareous spar, part of which would be dissolved by the superfluous acid. By the 

 quantity of spar remaining undissolved, it was learnt how much acid was required 

 to dissolve the magnesia. The iron and argillaceous earth contained in some speci- 

 mens, were precipitated by the spar, and therefore could not occasion any error. 



