_Antiscrofulitics_ form the third order. Iodine and its compounds have certainly an action on the blood. When given in excess, or for a long time, they may cause a train of symptoms to which the collective name of _Iodism_ has been applied. The most prominent are--coryza, itching, watering of the eyes, fever, and great constitutional depression. It powerfully promotes absorption in certain cases. Its special actions will be subsequently described in the remarks made on the second and third orders of Catalytic medicines. Potash has been already considered. Bromine and free Chlorine are very analogous to Iodine in their operation. _Antiarthritics_ exert an influence on the blood. This is least apparent in the case of Colchicum, which is a somewhat mysterious remedy. But it has been said by some to increase the amount of Urea in the urine; by others, to promote the excretion of Uric acid. (_Vide Antiarthritics, and Chap. IV._) Nitro-hydrochloric acid contains Chlorine, which is an oxidizing agent, and probably exerts in the blood an action of this kind. It is clear that it is capable of retarding the formation in the system, and deposit in the urine, of lithic acid and oxalate of lime; though its _modus operandi_ is not determined with certainty. This question is of some theoretical importance, and will soon be considered at some length. Among _Antiscorbutics_, Potash and its salts have been already shown to affect the blood. Their efficacy in scurvy is, however, a disputed point. That of Lemon-juice is established. It contains Citric acid and Potash; the former in great excess. Now either of these by itself would, in cases requiring it, and given in small doses, act as a Restorative. In large doses, Citric acid would disturb the reaction of the secretions, and Potash would impoverish the blood. When both together, the Citrate of Potash would undergo oxidation, and thus exert an operation in the blood of a refrigerant kind. Similar materials occur in most fresh vegetables. _Antiperiodics_ are medicines which are used in the cure of Ague, and of other periodic diseases, on the Catalytic principle. Tonics are used in the same diseases on the Restorative plan; but their utility extends also to other cases on the same grounds. Arsenic is the most potent of Antiperiodics. The Terchloride, and the Arsenite of Potash, are the preparations most frequently employed. Arsenic produces an evident action in the blood. When given in slight excess, it is apt to bring on [oe]dema of the face, and redness of the conjunctivæ. Besides causing various nervous disorders when in large doses, its continued use may bring on a cutaneous eruption, described by Mr. Hunt as a kind of pityriasis, together with a failure of appetite, general depression, a quick small pulse, hurried respiration, and sometimes swelling of the feet. Any kind of eruption would alone constitute sufficient proof of an operation in the blood.