312 
Mr. Tennant on different Sorts 
weight of each specimen. When the superfluous vitriolic acid 
had been evaporated by heat, the Epsom salt was separated from 
the gypsum by water. The result of these trials is expressed in 
the following table. 
Dry 
5 grains of limestone from Breedon gave 
gypsum. 
3-9 
Dry Epsom salt. 
315 
Matlock 
3-95 
2 9 
— Worksop 
3-8 
3-° 
York 
3-8 
3-i 
3 grains of calcareous spar 
of calcined magnesia gave 
As the preceding method of estimating the quantities of mag- 
nesia and calcareous earth is liable to considerable error, I after- 
wards 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 remaining 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 redissolve 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 ar- 
gillaceous earth contained in some specimens, were precipitated 
by the spar, and therefore could not occasion any error. 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 expert 
and 1 grain ] 
(3-9 
2.7 
