1836.] 



Note on the Magnestte. 



233 



the great difficulty of dissolving it, but subsequent observation proved 

 it to contain no silex. Its composition would seem to be, carbonic acid, 

 47*5 ; water 4*0* ; magnesia 48-5. As it is likely to become an article 

 Of commerce, and the statements regarding it are contradictory, 1 send 

 some for your re-examination. It occurs in thin veins (from an inch to 

 a foot), and also, (it is said,) in beds." 



" As the atomic weight of magnesia differs materially in different 

 chemical works, I was anxious to make use of this mineral to set the 

 mineral at rest, and decide whether Berzelius, Thomson, or Brande was 

 most to be trusted. 



" Three careful experiments proved, that the water contained was 

 0*8 per cent., while the slight adulteration of silica left, on dissolving 

 100 grains, was only 0*3 j traces of alumina and oxide of iron were 

 visible in the form of a delicate brown gelatinous film on adding 

 ammonia to the solution, but none of lime, even after adding sulphuric 

 or oxalic acid, evaporating to dryness, and re-dissolving in distilled 

 water. The solid impurities, therefore, being set against the gaseous, 

 as nearly in the proportions of the magnesian salt itself, it is evident, 

 that simple calcination of the solid mineral will give a very exact view 

 of its constituent proportions. 



" Ten specimens of 100 grains each, treated in this manner, returned 

 from the fire, weighing respectively, 49.67, 48.26, 48.20, 48.40, 48.40, 

 48.38, 48.39, 48.3 J, 48.37, and 48.33. The first of these was in the solid 

 form, and therefore may not have been thoroughly calcined : the aver- 

 age of the rest gives, 



Magnesia 48.34 by Berzelius 48.31 f 



~ Carbonic acid.... ,...5 1.66 51.69 



100 100 

 or almost precisely the composition according to this accurate chemist 

 —which it may be remembered was the only one which would agree 

 with my analysis of the Jabalpur dolomite, a definite crystallized com- 

 pound of one atom of carbonate of lime and one of magnesia. 



" To prove that no influential quantity of carbonic acid was retain- 

 ed, two of the specimens, were dissolved in dilute nitric acid, in a 

 closed glass tube — the gas extricated was less than the 50th of a cubic 

 inch. 



" The mineral was found to differ considerably in weight from the 

 Statements of Thomson and Phillips — the specific gravity of two 



* Dr. Malcolmson afterwards corrects this error. A part of the carbonic acid was 

 driven off with the water. 



t By Dr. Thomson, M. 46.2 C. A. 53.8 ; by Brande M. 47.2 ; C, A. 52.8, 



