216 
ON SOLUTION OF CITRATE OF MAGNESIA. 
precipitated at the expense of the bibasic portion, so as to leave the 
supernatant liquid more acid than at first. I have noticed this in 
but few instances, but in one of them, where a dozen bottles of 
the solution had precipitated largely, the liquid above the precipi- 
tate was excessively acid. This change may be accounted for by 
assuming 2(2MgO HO, Ci) to be converted into 3MgO Ci which 
precipitates, and MgO,2HO, Ci, which remains in solution and 
causes the acidity. 
In the formula of the United States Pharmacopoeia, the propor- 
tion of citric acid to magnesia is nearly that of a neutral salt, 
there being an excess of about seven grains of the acid, which, 
added to the seven and a half grains contained in the lemon syrup, 
gives the acidity to the officinal solution. There is not a sufficient 
excess of acid in these proportions to keep the solution long from 
precipitating, and hence the propriety of the revisers in directing 
the quantities for but one bottle or dose. The presence of the sy- 
rup in the solution retards the precipitation of the salt, and unless 
in winter, it will generally keep a week. 
In making enquiries among the apothecaries of this city as to 
the proportions of magnesia and acid they employ, I find the fol- 
lowing : 
Propor. in Neutral Citrate Acid 100 Magnesia 30 
« « U. S. Pharm. " 100 « 29,55 
« of A " 100 « 30 
« « B » 100 « 29,5 
« « C « 100 « 29,3 
« D « 100 « 29,1 
u u E « 100 «< 25 
The last solution, from the statement of the apothecary, keeps 
very well, which is probably due to its decided acidity. 
It is the custom with some pharmaceutists to make a dense so- 
lution of citrate of magnesia, pour the necessary quantity with the 
lemon syrup into the bottle, draw it full of carbonic acid water and 
immediately cork. Others derive the carbonic acid gas necessary 
to render the solution effervescent, from bicarbonate of polassa, 35 
grains of which salt in crystals is added just before the cork is se- 
cured. As this salt contains nearly half its weight of carbonic 
acid, and dissolves much sooner than the carbonate of magnesia, as 
