Dr Daubeny on separating Lime from Magnesia, 109 
Admitting such an affinity as we suppose to exist bet\v^een 
two earths, one of which it is our intention to disengage from 
its present combination, through the agency of another acid, it is 
very possible that if the divellent and quiescent attractions are 
already nearly balanced, the intervention of the third substance 
may altogether prevent a decomposition from taking place,, 
which, under ordinary circumstances, would have been effect- 
ed. 
Without putting so extreme a case, nothing is more likely 
than that the affinity between two bases, existing together in the 
same solution, should so modify the results obtained by the action 
of re-agents upon either of them, as to auginent or diminish 
materially the quantity of precipitate thrown down. 
That the latter case at least is not an hypothetical one, I shall 
have occasion to shew, in the course of the present paper, but 
there are abundance of facts already familiar to chemists, that 
<.*an only be explained on this supposition, such, for example, as 
the increased solubility (or the reverse) of salts in water, con- 
taining some third substance in solution, and the resistance which 
metals oppose to the action of acids, when united with certain 
other bodies *. 
• My friend the Rev. J. Coneybeare, has cornmiinicated to me the following 
fact, w^hich confirms the view I Imve taken of this subject. 
If chromate of lead and carbonate of potassa are fused, a double decomposi- 
tion is supposed to take place, chromate of potassa and carbonate of lead being the 
results, so that the former, from its solubility in water, might, according to this 
view of the case, be separated from the latter by repeated washing. Mr Coney- 
bear, however, finds that the yellow solution contains an admixture of chromate 
of lead, which may be separated by diluted nitric acid. After the separation of 
this precipitate, (the further addition of nitric acid producing no longer any such 
effect,) the chromic acid or chromate of potash may be obtained by evaporation in 
the mode usually prescribed. 
The fact does not appear to admit of a ready explanation, unless we suppose 
that the co-operating affinity of the chromate of potash for the chromate of lead, en- 
abled the water to take up a portion of the latter salt, for which it has not unas- 
sisted, an affinity powerful enough to overcome the cohesive attraction existing be- 
tween the particles of the substance itself. 
The circumstance of a portion of chromate of potash remaining undecom- 
pounded, might be adduced in favour of Berthollet’s views. 
Those who consider that entire insolubility among salts, is a property nearly as 
uncommon as perfect infusibility among bodies in general, will consider this fact 
analogous to those already w'ell ascertained, where water is enabled to take up a 
