280 
MUTUAL DECOMPOSITION. 
compositknis When the soluble salt has an insoluble base, the decompo- 
of t, t e insohr- sition does not meet with any obstacle, but continues until the 
nte neutral r . , 
salts, &c. n^uid contains nothing more in solution. The results of the 
experiment vary slightly according to the nature of the base of 
the salt made use of. If it be a salt of which the base is lime 
or magnesia, in proportion as the acid forms with the base of 
the insoluble carbonate a salt which is precipitated, the lime is 
precipitated also in a state of carbonate* 
If the base be metallic it almost always forms a salt with 
excess of oxide, which being insoluble is precipitated. 
AH these decompositions belong, with regard to theory, to 
some one of those which are already known ; and I have only 
related them, in order to particularise the characters which are 
presented by those which are acted upon by the salts, which have 
for their base potash and soda. 
The reaction of. the latter on the insoluble carbonates differs 
essentially from these of the preceding. A small quantity of 
carbonic acid is always disengaged at the commencement cf 
the Operation, but it soon ceases, though the decomposition 
don i/mqes to make progress. v No salt of this class is entirely 
decomposed by any carbonate. The quantity of the insoluble 
salt which is formed, is more or less considerable, according to 
the nature of the soluble salt and insoluble carbonate which is 
used 5 but when the reaction is no longer possible, there always 
remains a portion of the acid of the soluble salt in the liquid 
along with the greater ; part of the carbonic acid of the decom- 
posed carbonate. 
. . ,(Tq ly continu ed.J 
* When the sulphate of mngneda is decomposed by an insoluble 
carbonate, if a sufficient quantity of the latter be not employed, the 
fluid, after the operation, shews signs of alcalinity, and it may be sup- 
posed that by thus separating a portion of the sulphuric acid from the ' 
magnesia, the result would be a salt with an excess of the base. But 
this phenomenon depends upon the sub-caibonate of magnesia being 
soluble in a solution of the Milphate of the same base. This property 
furuishes a very convenient method of freeing the sulphate of mag- 
nesia from the influence of the iron and manganese which it eon- 
tains, 
A Mem oh 
