Water, on account of its high specific heat, is the most valuable agent for applying heat or cold to the surface of the body or to the internal organs. The cold-water bag or ice bag and the hot-water bag are well-known weapons in the armament of sane therapy. The effect of cold in certain conditions of the brain and in certain high temperatures due to disease is well known. The reduction of the temperature, while only a temporary relief, is necessary to the proper action of the various medicinal healing agents.

The application of cold by means of water or ice is justified on many therapeutic principles. As is well known, a low temperature interferes with the activity of vital forces. In localized inflammation, turgidity and attendant pain, the judicious application of cold may afford at least temporary relief. In cases where the activity of the cells is for the purpose of excreting all poisonous substances cold must be used with careful judgment, or else it may do serious harm. In such cases the application of hot water may result in greater good. The temporary contraction of the blood-vessels after cold water or ice is applied is always followed by a reaction, when it is removed, of greater activity. When one has been out in the cold during the day his cheeks are apt to become very rosy on entering a warm room at night. Thus if increased activity and secretion are desired in the end, it may be justifiable in the beginning to apply the cold compress.