Remedies which obviate acidity in the stomach, by combining with the acid, and neutralizing it. This is the most decided instance of chemical action which occurs in the history of medicinal operations. We have an acid whose presence excites morbid symptoms in the _primæ viæ_, and these are immediately removed by the administration of any one of those substances which are capable of forming a natural compound with the acid in question, _out of the body_, and the same proportions are required in both cases for saturation. If a carbonated alkali be employed, the same disengagement of carbonic acid takes place in the stomach as would occur in the laboratory, and a new compound is produced, whose operation varies according to the chemical nature of the substance employed; thus, the salt which magnesia forms with the acid in the stomach proves slightly purgative, while that which lime produces under similar circumstances is distinguished by an opposite property. ANTILITHICS AND LITHONTHRYPTICS. ANTILITHICS[182] are remedies which have the power of preventing the formation of those mechanical deposites from the urine, which give origin to calculous concretions; and may belong either to the class of Vital, or Chemical Agents. LITHONTHRYPTICS[183] are those medicines which, by a chemical operation, are capable of dissolving calculous concretions. It has been already shewn, while treating the subject of Diuretics, that certain substances, when internally administered, are capable of passing the barriers of digestion, and of entering the circulation; and that, moreover, these bodies may be again separated by the secretory vessels of the kidneys, and be ejected from the body in the urine. It cannot therefore be contended, that the urinary calculus is placed beyond the sphere of direct medicinal influence, nor can any argument, founded upon the alleged incompatibility of chemical and vital action, be fairly maintained in this case; for the urinary calculus, as well as the urine itself,[184] may very justly be considered as extraneous to the living body. The existence of such a class of remedies as that of Lithonthryptics being thus established, we have to consider the mode and possible extent of their operation in the different varieties of the disease, which they are thus calculated to palliate or cure. In entering upon this inquiry, it is not my intention to prosecute the subject farther than may be necessary to explain the _modus operandi_ of the remedies in question, and in conformity with the object and plan of this work, to establish some general principles that are to direct us in their election, combination, and administration; for farther details the practitioner must consult the systematic treatises of _Prout_[185] and _Marcet_,[186] and the very able papers of Mr. _Brande_,[187] and Dr. _Wilson Philip_.[188]