This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
This is very frequently recommended and also employed, but does not deserve that credit which is given to it. Its employment renders certain syrups, especially when some alkaloids enter into its combination, absolutely unfit. Syrups treated with carbonate of magnesia acquire an alkaline reaction, which is obnoxious or even trouble-some, and is injurious to the delicate flavors used in compounding syrups. Calcined magnesia should be rejected for the same reasons, as its action is similar. When acidified syrups, such as prepared with citric or tartaric acid or fruit syrups, are to be clarified, magnesia in no form can be used.