210 ENERGY our power to perform the inverse operation, and to utilize the whole of the heat in doing mechanical work. Thus we see that different forms of energy are not equally valuable for conversion into work. The energy of a system should be measured by the amount of work it can do under the most favourable conditions which can be imagined, though we are not necessarily capable of realizing them. The ratio of the portion of the energy of a system which can under given conditions be converted into work to the whole amount of energy present is called the availability of the energy. If a system be removed from all com munication with anything outside of itself, the whole amount of energy possessed by it will remain the same, but will of its own accord tend to undergo such trans formations as will dimmish its availability: for since work is done only when energy undergoes transforma tion, every change which itis allowed to undergo of its own accord deprives us of one opportunity of deriving useful work, that is, of converting a portion of the energy into the particular form we desire. This principle, known as the principle of the dissipation of energy, was first pointed out by Sir William Thomson in the Philosophical Magazine for April 1852, and was applied by him to some of the prin cipal problems of cosmical physics. Though controlling all phenomena of which we have any experience, the principle of the dissipation of energy rests on a very different founda tion from that of the conservation of energy ; for while we can conceive of no means of circumventing the latter principle, it seems that the actions of intelligent beings are subject to the former only in consequence of the rudeness of the machinery which they have at their disposal for con trolling the behaviour of those portions of matter in virtue of the relative motions or positions of which the energy with which they have to deal exists. If we have a weight capable of falling through a certain distance, we can employ the system consisting of the earth and weight to do an amount of useful work which is less than the potential energy possessed by the system only in consequence of the friction of the constraints, so that the limit of availability in this case is determined only by the friction which is unavoidable. Here we have to deal with a system with which we can grapple, and whose motions can be controlled at will, If, on the other hand, we have to deal with a system of molecules of whose motions we become conscious only by indirect means, while we know absolutely nothing either of the motions or positions of any individual molecules, it is obvious that we cannot grasp single molecules and control their movements so as to derive work from the system. All we can do, then, is to place the system under certain conditions, and be content with the amount of work which it is, as it were, willing to do under those conditions. It is well known that a greater pro portion of the heat possessed by a body at a high tempera ture can be converted into work than in the case of an equal quantity of heat possessed by a body at a low tempera ture, so that the availability of heat increases with the temperature. Clerk Maxwell supposed two compartments, A and B, to be filled with gas at the same temperature, and to be separated by a partition containing a number of trap doors, each of which could be opened or closed without any expenditure of energy. An intelligent creature, or " demon," possessed of unlimited powers of vision, is placed in charge of each door, with instructions to open the door whenever a particle in A comes towards it with more than a certain velocity V, and to keep it closed against all particles in A moving with less than this velocity, but, on the other hand, to open the door whenever a particle in B approaches it with less than a certain velocity v, which is not greater than V, and to keep it closed against all ! particles in B moving with a greater velocity than this. By continuing this process every unit of mass which enters B will carry with it more energy than each unit which leaves B, and hence the temperature of the gas in B will be raised and that of the gas in A lowered, while no heat is lost and no energy expended, so that by the application of intelligence alone a portion of gas of uniform pres sure and temperature may be divided into two parts, in which both the temperature and the pressure are different, and from which, therefore, work can be obtained at the expense of heat. If the gas do not liquefy, there seems no limit to the extent to which this operation may be carried, by increasing V and diminishing v, except that v cannot be made less than zero, which corresponds to the whole of the energy being abstracted from the gas in A and given to that in B. This shows that the principle of the dissipation of energy has control over the actions of those agents only whose faculties are too gross to enable them to grapple with those portions of matter in virtue of the relative motions or relative positions of which the energy exists with which they are concerned. In April 1875 Lord Rayleigh published a paper in the Philosophical Magazine on " the work which may be gained during the mixing of gases." In the preface to the paper Lord Rayleigh says, " Whenever, then, two gases are allowed to mix without the performance of work, there is dissipation of energy, and an opportunity of doing work at the expense of low temperature heat has been for ever lost." He then shows that the amount of work obtainable is equal to that which can be done by the first gas in expanding into the space occupied by the second (supposed vacuous) together with that done by the second in expanding into the space occupied by the first. In the experiment imagined by Lord Rayleigh a porous diaphragm takes the place of the partition and trap-doors imagined by Clerk Maxwell, and the gases sort themselves on account of the difference in the velocities of mean square of molecules of the different gases. When the pressure on one side of the diaphragm is greater than that on the other, work may be done at the expense of heat in pushing the diaphragm, and the operation continued until the gases are uniformly diffused. There is this difference, however, between this experiment and Clerk Maxwell s, that when the gases have diffused the experiment cannot be repeated, and it is no more contrary to the dissipation of energy than is the fact that work may be derived at the expense of heat when a gas expands into a vacuum, for the working substance is not finally restored to its original condition ; while Clerk Maxwell s experiment may be supposed to be continued and work obtained till the whole of the gas has been reduced to the absolute zero of temperature, and the ex periment may be repeated by again heating the gas. Inde pendently of Lord Rayleigh, Mr S. Tolver Preston, in November 1877, called attention to the work which may be done at the expense of heat during the diffusion of gases, and the bearing of this upon the dissipation of energy (see Nature, Nov. 8, 1877). In these experiments the molecular energy of a gas is converted into work only in virtue of the molecules being separated into classes in which their velocities are different, and these classes then allowed to act upon onc 4 another through the intervention of^ a suitable heat engine. If we could carry out this subdivision into classes as far as we pleased we might transform the whole of the heat of a body into work. The availability of heat is limited only by our power of bringing those particles whose motions constitute heat in bodies to rest relatively to one another; and we have precisely similar limits to the availability of the energy due to the motion of visible and tangible
bodies.