250 M. Berzelius on some Comjyounds 
water must enter into this compound, in a number proportion- 
ally greater than those of the carbonic acid. In all double salts, 
which have a common base and different acids, it is probable 
that the weaker of those acids must exist at a point of satura- 
tion proportionally inferior to that of the stronger acid : exact- 
ly as in double salts which have two bases, the weaker of those 
bases is often less saturated with acid than the stronger base. It 
is possible, therefore, that when a carbonate combines with a 
hydrate to form a double salt, the carbonate may contain no 
water of crystallisation, and that all the water found in this 
double salt may belong to the hydrate. In magnesia alba, the 
carbonic acid contains two times the oxygen belonging to that 
part of the magnesia combined with it ; and the water contains 
four times the oxygen belonging to that part of the magnesia 
which forms the hydrate ; so that the w^ater, in this case, is 
at a degree of saturation proportional to the bicarbonate of 
this earth. According to these premises, magnesia alba is 
Mg Aq^ -j- 3 Mg C^, from which the composition, deduced 
by calculation, is, 
Magnesia, - - 44.641 
Carbonic acid, - 35.736 
Water, - - 19.621 
which agrees perfectly with the result of our analysis. 
Carhonatt of Zmc. -^The carbonate of zinc is known to be 
soluble in water, by an excess of carbonic acid ; and when the 
sulphate of zinc is decomposed by an alkaline carbonate, a great 
quantity of the precipitated oxide remains dissolved in carbonic 
acid. The resemblance of this phenomenon, to what happened 
in the case of magnesia alba, gave rise to the suspicion that it 
was owing to a similar cause, and induced me to submit the car- 
bonate of zinc to a particular examination. I then found that 
this carbonate has the same composition when precipitated in a 
cold, as in a boiling, state ; and consequent!}^ that the usual 
quantity of water, employed in such an experiment, is suffi- 
cient to complete its decomposition. The carbonate of zinc, 
when precipitated from a solution containing also muriate or 
sulphate of zinc, always carries with it a portion of muriatic or 
sulphuric acid, which washing cannot remove. When precipi- 
3 
