Sea-bathing is a very popular form of the natural bath, and it is preferable to bathing in river-water or spring-water, because the sea is seldom so cold as are the latter. A sea-bath has also another great advantage over all other forms of bath, and that is that it is taken in the purest air possible; and in considering the effects of sea-bathing it is impossible to separate the effects of sea-air from that of the sea-water. The sea-bather is also constantly inhaling the spray of the sea-water, and thus obtains whatever benefit is to be got in this way. If he can swim he enjoys all the benefit of exercise. The motion of the water and the buffeting he gets from the waves act as a powerful excitant to the skin, and lastly, the salt in the water adds considerably to the stimulating action. Reaction more readily occurs after a sea-bath than after a river-bath, and thus the liability to “catch cold” is less, although the popular belief that it is impossible to take cold from a wetting with salt water is far from being true. Besides the water of the sea, there are many other _natural salt waters_ which have a great reputation both for bathing and drinking. These salt waters, which may be got of all strengths, from a strong natural brine to a water in which the salt is scarcely recognisable, all owe their stimulating power, as does sea-water, to the chloride of sodium (common salt) and other chlorides which they contain. Salt-water baths, or sool baths as they have been called, act as powerful stimulants to the skin, and have a very great reputation in Germany and other places, where the only seaboard is the ungenial northern coast. There are many _natural mineral waters_ which contain ingredients other than common salt, and all of these are much used for bathing. We shall give some details of these when we come to speak of bathing resorts, and at present we shall content ourselves by giving merely a rough classification of them. 1. Many waters issue from the ground at a temperature sufficiently hot, or even too hot (e.g. the geysers in Iceland) for bathing. Some of these natural hot waters contain very small quantities indeed of mineral matters, and these are known as _indifferent thermal springs._ 2. Mineral waters containing common salt have been already alluded to. They are known as _salt springs._ 3. The so-called _alkaline springs_ contain as their chief ingredient carbonate of soda. These waters are more used for drinking than for bathing. The alkali which they contain helps undoubtedly to soften the skin of the bather, and acts probably also as a stimulant to the surface. 4. The waters containing bitter _purgative salts_, such as Epsom salt or Glauber’s salt, owe their reputation almost entirely to their power when taken internally. When used for bathing it is probable that these natural solutions of purgative salts are more stimulating to the skin than ordinary water.