5. The natural _chalybeate waters_, or waters containing iron, are but little used for bathing, and it is exceedingly unlikely that the iron contained in the water has any effect upon the bather. Although we have classified the waters, and have used that classification which is generally adopted, it must be remembered that the ingredients of waters are always multiple, and we usually find that they contain alkaline salts, purging salts, iron salts, and brine salts mixed together, so that it is difficult sometimes to determine which is the predominating ingredient, and therefore to which class a mineral water properly belongs. All water contains gas of some kind dissolved in it, and it is well known that a glass of ordinary spring water may be seen to contain bubbles of gas which adhere to the sides of the glass, or come “winking at the brim.” The gases which waters principally contain are atmospheric air, nitrogen, carbonic acid, and sulphuretted hydrogen, and great stress has been laid upon the presence in bathing waters of the three last-named gases. The action of bubbles of gas contained in water is, in part at least, easy to understand. These bubbles give great mobility to the water, and thus the particles of water in contact with the skin are incessantly changing. Gas is soon driven out of water by the application of heat, and it is only the cooler of the thermal springs which remain charged with any considerable quantity of gas after the natural pressure, to which they have been subjected in the earth, has been removed.