Dr. A. T. Thomson states that the action of Arsenic is liable to exacerbations and remissions, and sometimes even intermissions. Thus we may suppose that there is a certain degree of analogy between its operation and that of the malarious poison, by virtue of which it may perhaps exert a corrective power over the working of the latter in the blood. (_Vide_ page 166.) Such an analogy could only exist in the case of a Catalytic medicine. It is not observed of Quina, which is employed in intermittents on the Restorative plan. Arsenic is foreign to the blood, and is in every way a Catalytic medicine. In large doses Arsenical preparations act on the stomach as irritant poisons. Some of them have been used externally as Caustics, especially in Lupus. But this is a kind of action that does not concern us now. The officinal solution of Arsenite of Potash was introduced into notice by Dr. Fowler, and is the chief preparation of Arsenic employed. De Valangin's solution contains a Terchloride of Arsenic, and is of less than half the strength of the others. Both of them are used in Ague. Some recommend, in all cases where Arsenic is used, to begin with small doses, as two drops of Fowler's solution three times a day, and gradually to increase the quantity; from the idea that the system will thus be enabled to tolerate the remedy better, and that irritation of the stomach will be avoided. Mr. Hunt deprecates this practice, and recommends regular doses of three to five drops or more. It may be safely continued during the paroxysm of Ague, which is a plan that is generally considered to be unadvisable in the case of Quina. The latter may be given in a very full dose between the paroxysms; but this cannot be done with Arsenic, for the consequences would be dangerous. It is best in all cases to give the Arsenic after a meal, on a full stomach, as then it is less likely to irritate, not coming directly in contact with the coats of the stomach, and being diluted by the food during its absorption. Arsenic is useful in other intermittent disorders besides Ague, as in the various kinds of intermittent neuralgia. It has been given in some varieties of intermittent pulse, which are not due to organic disease of the heart. Dr. Darwin cured a case in which the beats of the heart intermitted regularly once in every three or four times, by the administration of four drops of a saturated solution of Arsenious acid three times a day. The preparations of Arsenic have even been used in some cases of Gastrodynia with considerable advantage. In these instances the disorder of the stomach generally assumes an intermittent form. It has been supposed by some that Arsenic in such disorders acts directly upon the nerves; but it appears to counteract all the manifestations of this intermittent poison, whether local or general, by the same action in the blood. (_Vide_ page 212.)