^16 Dr Daubenj mi separcf^ting Lkae from Magnesia. 
found, that after they had been boiled for hours with mag- 
nesian solution, although the alkali continued at the end of 
the experiment to be in excess, the presence of a larger portion 
of magnesia was detected in the filtered liquor, by adding car- 
bonate of ammonia with a phosphoric salt, than could be ac- 
counted for, from the solubility of magnesia and its carbonate in 
the water. 
Nevertheless, I am ready to allow, that, by adding a consider- 
able ex<;ess of the reagent, soda, and still more effectually 
potass, together with their respective subcarbonates, may suc- 
ceed in throv,^ing down the whole of the magnesia present ; for 
the smallest quantity of the latter dissolved in water, or united 
with an acid, gives a flocculent precipitate in solutions of these 
alkalies. It is right, however, that we should be aware, that 
the existence of a slight excess of the reagent in the solution, is 
no sufficient evidence that the whole of the earth- has been sepa-. 
rated. 
There is another objection to the use of these reagents, where 
extreme exactness is required, namely, that the precipitate caused 
by them, often appears to retain a trace of the acid with which 
it was originally united. 
I have the authority of Berzelius for stating this fact, with 
regard to the precipitate afforded by ammonia ; and when the 
nnagnesia was thrown down from its sulphate by pure soda, I 
have found, that, on being redissolved in muriatic acid, it continu- 
ed to give decided indications of the presence of sulphuric acid, 
even when the distilled water, wuth which it had been washed, 
ceased to produce the slightest alteration in a solution of muriate 
of barytes. 
These experiments, however, must be repeated, before I can 
feel myself warranted in arriving at any fixed conclusion, on a 
point which involves so important a question of chemical theory ; 
but it may be useful to mention the results I have obtained, 
however insufficient they may be to decide the question, if it 
from 50 gr. of crystalline sulph. magnesia, containing 8.0 of base, was 6.9 
taking the mean of three experiments ; by subcarbonate of soda, 7.2. The preci- 
pitates by potash and its subcarbonates seemed to be somewhat larger; but the 
sparing solubility of sulphate of potash, renders the employment of the latter re- 
.agents to throw down magnesia from its sulphate somewhat inconvenient 
