Intelligence and Miscellaneous Articles. 475 



if the attention be momentarily directed to the appearance of the 

 needle, its outline is seen to be indistinct, and those parts which re- 

 flect most light are seen with luminous protuberances, exhibiting all 

 the appearances of light imperfectly focalized on the retina. If the 

 eye be kept directed to the needle for but a very short time (accom- 

 panied perhaps by an effort to see distinctly), the ill-defined image 

 rapidly contracts to perfect vision. 

 Topsham, near Exeter, March 4, 1844. 



DOUBLE CARBONATE OF AMMONIA AND MAGNESIA. 

 BY M. P. A FAVRE. 



This salt may be obtained in several modes ; by agitating carbo- 

 nate of magnesia in a solution of carbonate of ammonia and filtra- 

 tion, the solution rapidly deposits crystals on the sides of the vessel 

 containing it, and these are right rectangular prisms ; when they are 

 collected and spread on filtering paper, they dry rapidly ; after being 

 inclosed in a bottle, when perfectly dry, they did not yield any am- 

 moniacal odour, on opening the bottle several days afterwards. 



The portion remaining on the filter, again submitted to similar 

 treatment, yielded a solution which afforded more crystals, and even- 

 tually it altogether assumed a granular aspect, and became entirely 

 the salt in question, of which large quantities may be thus obtained. 

 The analyses which were made of this salt were performed upon 

 crystals which were deposited from the filtered liquor, and not those 

 which granulated. 



Another method for obtaining large quantities of these crystals, 

 and in a very pure state, consists in mixing a saturated solution of 

 bicarbonate of magnesia with one of carbonate of ammonia; the 

 double carbonate of magnesia, being very slightly soluble, notwith- 

 standing a great excess of carbonate of ammonia, but few crystals 

 are obtained by the first method above described ; whereas the bicar- 

 bonate of magnesia being more soluble, brilliant and well-defined 

 crystals of the double carbonate are obtained in a few seconds after 

 it is mixed with the solution of carbonate of ammonia. 



This salt in fine prismatic crystals yielded by analysis as under : — 



Magnesia 15-77 1592 158 



Carbonic acid . . 34-90 35*00 



Hydrogen 6-70 



Azote 11-60 



Oxygen 31-03 



100* 

 These analyses indicate the annexed formula : 



(CO 2 , MgO, C0 3 H 4 AzO, 4HO), ' 

 which gives the following numbers : — 



Magnesia 16*28 



Carbonic acid .... 34*70 



Hydrogen 6*31 



Azote 11*17 



Oxygen 31*54 



100* 



