To the Editor:—The popular antacid, magnesium trisilicate, is a hydrate of variable water content, a pharmaceutical fact that creates confusion. The U.S. Pharmacopeia defines magnesium trisilicate as "a compound of magnesium oxide and silicon dioxide with varying proportions of water." The only specification on the water content is an upper limit of 34%. The form generally used for medicinal purposes contains about 32% of water, on the average. However, the U.S.P. standard for acid-consuming capacity is expressed in terms of theanhydrouscompound. Thus, the attribute of greatest clinical significance is accurately standardized and commerce in the bulk form is on an adequately uniform basis. That is, each gram of water-free magnesium trisilicate must neutralize between 140 and 160 ml. of 0.1 N hydrochloric acid.The prevailing misunderstanding stems from the practice followed in formulating and labeling the products, which are mostly mixtures, that contain magnesium trisilicate. Inquiry revea