It is obvious that real measurable physiological changes can be produced by means of hydrotherapeutic procedures. This is doubtless the reason that hydrotherapy, although one of the oldest therapeutic agents, is still recognized as important by those who are familiar with its application. In the first place, it is readily accessible and may be applied with relatively simple and inexpensive equipment. It possesses the ability to absorb and communicate large quantities of heat. This is a relatively high specific heat as compared to a number of other common substances.
Water is also a good conductor of heat.
When the state of water is changed from fluid to vapor, about 540 calories of heat are required for each gram of water vaporized. This is known as the latent heat of vaporization of water, which is defined as the heat required to vaporize one gram of it at a constant temperature. Because of this, the evaporation of water or perspiration from the skin surface has a pronounced cooling effect proportional to the rate of evaporation. When water vapor or steam changes back to liquid, about 540 calories of heat are liberated. This is illustrated by the severe burns which are produced when live steam comes into contact with the skin and condenses on it.