1835.] Native Carbonate of Magnesia. 511 



the Society, are several lumps of this curious mineral. Dr. M. writes: 



" The native carbonate of magnesia from Salem has again attracted 

 attention. I at first supposed it to be a magnesite, from the great 

 difficulty of dissolving it, but subsequent observation proved it to con- 

 tain no silex. Its composition -would seem to be, carbonic acid 

 47 5 ; water 40* ; magnesia 48*5. As it is likely to become an article 

 of commerce, and the statements regarding it are contradictory, I 

 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 

 matter 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 grs., was only 0*3 ; traces of alumina and oxide of iron were visi- 

 ble in the form of a delicate brown gelatinous film on adding ammo- 

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

 oxalic acid, evaporating to dryness, and redissolving 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 sim- 

 ple calcination of the solid mineral will give a very exact view of its 

 constituent proportions. 



Ten specimens of 100 grs. each, treated in this manner, returned 

 from the fire, weighing respectively, 49'67, 48-26, 48-20, 48-40, 

 48-40, 48-38, 4839, 48-33, 48-37, and 48"38. The first of these 

 was in the solid form, and therefore may not have been thoroughly 

 calcined : the average of the rest gives, 



Magnesia,. . . . 4834 by Berzelius 48-3 If 

 Carbonic acid, 51*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 compound 

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



To prove that no influential quantity of carbonic acid was retained, 

 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. 



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

 acid was driven off with the water, 

 f By Dr. Thomson, M. 46-2 C. A. 53-8 ; by Brande M. 47*2 ; C. A. 52*8. 

 3 u 2 



