Of these, magnesia itself, its carbonate, and sometimes its bicarbonate in solution, are used as antacids; but they have been already considered so fully that little else remains than simply to indicate that they belong to the class. One or two remarks, however, may be made here with propriety. The combining number of magnesia, with the exception of that of lithia, the relative scarcity of which puts it out of view in the question, and that of ammonia, which is precluded from the comparison, in ordinary cases, by its powerfully stimulant property, is the lowest on the list of the antacids, and its neutralizing capacity, consequently, the highest. Hence, when the object is to neutralize a large quantity of acid in the alimentary canal, this is the article preferred. its laxative property, after saturation, gives it also great advantages in certain cases. The general rule may be laid down, that, whenever there is occasion at the same time for a laxative and antacid, magnesia, or one of its carbonates, should be selected.