At the period when the second edition of this Treatise was prepared for the press, bromine and its compounds, though employed to a certain extent in medicine, had not obtained an importance which, in the opinion of the author, called for their introduction into the work as valuable articles of the Materia Medica. Their close chemical analogy with iodine and its compounds naturally suggested that they might exercise a similar influence on the system; and this conjecture was in some degree justified on trial. Bromine was found like iodine to possess alterative properties which were believed to render it useful in scrofulous and chronic rheumatic affections, particularly in the resolution of tumours, for which it was employed to some extent; but confessedly it was decidedly inferior to iodine in these respects, and seemed to be passing out of use, when the discovery of other and altogether unexpected virtues brought it anew into notice; and it is now among our most popular remedies. There are three prominent forms in which it has been used; 1. that of bromine in its pure elementary state, 2. that of bromide of potassium, and 3. that of bromide of ammonium; of which the first two are recognized in the U. S. Pharmacopoeia. I shall treat of these separately; and, under bromide of potassium, which is much the most used of the three, shall treat mainly of the therapeutic properties and applications of this set of medicines.