Another principle, almost equally general in its applicability, is that of the dissipation of energy, for which we are indebted in the first instance to Sir William Thomson. All forms of energy may be converted into heat, and heat tends so to diffuse itself throughout all bodies as to bring them to one uniform temperature. This is its ultimate state of degradation, and from that state no methods with which we are acquainted can transform any portion of it. When energy is possessed by a system in consequence of the relative positions or motions of bodies which we can handle, and whose movements we may control, the whole of the energy may be employed in doing any work we please; in fact, it is all _available_ for our purpose, or its _availability_ may be said to be perfect. Energy in any other form is limited in its availability by the conditions under which we can place it. For example, the energy of chemical action in a battery may be used to produce a current, and this to drive a motor by which mechanical work is effected, but some of the energy must inevitably be degraded into the form of heat by the resistance of the battery and of the conductor, and this portion will be greater as the rate of doing work is increased. The ratio of the quantity of energy which can be employed for mechanical purposes with the means at our disposal, to the whole amount present, is called the _availability_ of the energy. All forms of energy may be wholly converted into heat, but only a fraction of any quantity of heat can be transformed into higher forms of energy, and this depends on the temperature of the source of heat and of the coldest body which can be employed as a condenser, being greater the greater the difference between the temperatures of the source and condenser, and the lower the temperature of the latter. In every operation which takes place in nature there is a degradation of energy, and though some portion of the energy may be raised in availability, another portion is lowered, so that on the whole the availability is diminished. Thus, in the case of the heat-engine, work can be obtained from heat only by allowing another portion of the heat to fall in temperature; and, as originally stated by Sir William Thomson, "it is impossible, by means of inanimate material agency, to obtain mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects," and to leave the working substance in the same condition in which it was at the commencement of the operations. Accepting this principle, Professor James Thomson showed that increase of pressure must lower the freezing point of water, for otherwise it would be possible to construct an engine which, working by the expansion of water in freezing, would continue to do work by cooling a body below the temperature of any other body available, and he calculated the amount of pressure necessary to lower the freezing point through one degree.