S06 Dr DauBeny on separating Lime from Magnesia^ 
In conclusion, then, I may observe, that thb method we haV^ 
been considering, answers most beautifully the purpose of as- 
certaining the presence of minute quantities of magnesia, and 
that it may be also employed as a ready though not a perfectly 
accurate method of determining the proportion in which this 
base occurs, even when lime is present ; but that if we wish 
likewise to ascertain the amount of the latter earth, we shall find 
it necessary, before we add the carbonate of ammonia, to throw 
down the lime by some other means. Unless, therefore, we can 
discover some precipitant which will do this completely, without 
alfecting the magnesia the formula first mentioned seems of 
the two the most precise. 
III. M. Longchamp (whose paper in the 12th volume of the 
Annales de Chimie I have already alluded to) proposes to em- 
ploy ammonia in the state of a subcarbonate, for the purpose of 
separating the two earths, conceiving that we shall in this man- 
ner be enabled to throw down the lime before the magnesia be- 
gins to be separated. He asserts, that if the solution be filtered 
within two hours after the addition of the subcarbonate of am- 
with the nitro-phosphate of lead, which Berzelius mentions having obtained by a 
similar process, and of which 20.2 grains would be composed, according to his es- 
timate, of 13.47 phosphate; of lead, and 6.73 nitrate of lead. Now, 13.47 grains 
of phosphate of lead contain only 3.36 of acid, so that it is evident that all the 
phosphoric acid was not precipitated in this manner, for 10 grains of phosphate of 
magnesia contain about 6 gr. of acid. Time has not allowed me to prosecute the 
subject further. 
* This part of my paper was completed, when I was favoured by a letter from 
Mr H. Warburton, Vice-President of the Geological Society, in which he informs 
me, that Dr Wollaston considers it advisable, in the first instance, to precipitate 
the lime by oxalate of ammonia, taking care not to leave an excess of the oxalate, 
and not to have the solution of the lime too dilute after the oxalate of ammonia i& 
separated. Then to the clear liquor he adds bicarbonate of ammonia, and, lastly, 
phosphate of soda. “ The smallest quantity of magnesia will thus be rendered vi- 
sible, if the side of a watch-glass containing some of the liquid be touched below 
the surface of the fluid with a glass-rod.” I have had no direct experience of this 
last method, but can readily believe it to be an improvement upon the one usually 
employed by me. There could be little difficulty, however, in separating the two 
earths, if oxalate of ammonia could be employed, without acting upon magnesia, or 
being influenced by its presence. This, however, I shall endeavour to shew is of- 
ten the case. 
