Dr Daubeny 07i separating Lime from Magnesia. 123 
it I prefer calculating the quantity of magnesia from the re- 
siduum, after exposure to a red heat, taking it in both instances 
at 40 per cent, of the precipitate. 
The analysis, therefore, by phosphate of soda, indicates 20.3 
gr. of phosphate of magnesia, equivalent to 8.1 of the pure 
earth ; that by phosphate of ammonia 19.7 gr., or 7.88 of base. 
Now, we have stated, on the authority of the latest experi- 
ments, that 50 gr. of crystallised sulphate of magnesia contain 
8.0 of base, which is nearly the mean between the two analyses. 
To satisfy myself still further that the whole of the magnesia 
is thrown down by this re-agent, I observed whether potash, 
which (as I shall afterwards mention) is one of the most delicate 
tests of magnesia, produced any effect on a magnesian solution, 
after the alkaline phosphate had ceased to act ; but although 
this re-agent was added in considerable quantity to water that 
had contained solutions both of sulphate and muriate of magne- 
sia, the liquor remained, in either case, just as transparent as 
before. 
( To be concluded in next Number.) 
cgrding to the calculation made by him of the proportion between the amount of the 
base present in the precipitate, before and after exposure to a red heat. For, as 
he estimates the former at 19, and the latter at 40 per cent, of the whole, the pre- 
cipitate by phosphate of soda ought, in that case, to have been reduced from 45.8 
gr. only to 21.75 ; and, the one obtained by phosphate of ammonia from 44.5 to 
21.14. For, as.40: 19; ;45.8: 21.75; and as 45.8 : 21.75 : : 44.5 : 21.14. Where- 
as the residuum in the foriner was less by 1.45, and in the latter by 1.44. The 
circumstance that the two precipitates were reduced by a red heat, in exactly the 
same ratio, (corresponding to the 3 ; ^ ^ grain), whilst it gives me some confi- 
dence in the accuracy of the results themselves, seems rather to favour the idea 
that the quantity of ammonia present exceeds Dr Marcet’s statement, for it is not 
likely, that the : pr^pitates, after remaining under an exhausted receiver, should 
have both retained precisely the same proportion of water. I ^am 'unwilling, how- 
ever, to call in question the experiments of so accurate a chemist, without having 
more solid grounds to go upon. 
* I have found that the triple phosphate begins to be decomposed at a tempe- 
rature very little exceeding 100° of Fahrenheit. 
