light, the only difference between light, heat, and electricity consists in the length of waves in the ether of space. The sun is the source of electro-magnetic waves, and the earth is the scene of transformations of electric energy. A piece of coal burning in a grate has therefore a long electro-magnetic history. It owed its origin to electro-magnetic waves, and in burning it gives out again electro-magnetic waves, of which we can only detect the light and heat manifestations.

The Rumford professorship in Harvard University was endowed by Count Rumford in order to promote our knowledge of light and heat; for many years the lectures given under the endowment were devoted exclusively to the subject of light and heat and the conservation of energy-a subject to which Count Rumford's experiments may be said to have given one of the primal impulses. The lecturer and student to-day, however, is compelled to approach the subject of light and heat through the broadest study of electromag

netism.

Tyndall, I have said, in his Heat considered as a Mode of Motion, has happily illustrated the great doctrine of the conservation of energy. No truth is better established in physics than that of the equivalence between work and the heat produced, and the modern developments of electricity afford means for richly illustrating it. The claims of the great founders of this hypothesis have been set forth by Tyndall in his treatise, and by Tait in his Recent Advances in Physical Science. My object in this treatise is to call attention to the transformations of energy rather than to discuss the claims of priority; to follow the protean forms under which energy manifests itself, rather than to measure the equivalence of these forms of energy.

If we wish at this stage of our presentation of the subject of the transformation of energy exultantly to proclaim the results of the methods of the measurements of heat equivalents of motion, we can point to the commercial applications of electricity, which are all based upon exact experiments on the equivalence between the heat produced by the current and the work done by the steam engine which produces the current.

The doctrine of the conservation of energy is little more than a hundred years old. Count Rumford made his celebrated measurement of the heat produced by the boring of a cannon in 1789. Yet in this time the world, working with this powerful theory, has made greater progress than it did with its physical speculations during the comparatively immense historic period which preceded 1789.

Not only in the commercial world do we find that the great physical doctrine has accomplished great results. In the subject of medicine the doctrine is daily being recognised; for the application of external heat to the human body to diminish the effort of the human organism to supply heat, or to supplement this effort in very young children or in very old people, is now clearly understood. The death of very old persons at night is often due to the want of heat. They can be said to freeze to death. Very young children often perish because sufficient heat is not supplied to them. It seems, therefore, to be both physically and physiologically unphilosophical to expose the limbs of young children to the cold air. In hospitals, during severe operations, the patient is placed upon a warm table and is afterward surrounded with hot-water bags or similar contrivances to supply heat. This external heat facilitates the various transformations of energy which are

going on in the human organism, and too great a demand is not made upon the internal mechanism to supply this heat.

The study of the transformations of energy in the human body is a far more difficult one than the study of the duty of an ordinary steam engine, for the transformations are more numerous and subtle. In the main, however, there is a close relation between the amount of food consumed and the work that a man can do.

The most powerful instrument for studying the transformation of energy is still the steam engine. By means of this great invention of Watt we obtain our electrical currents; and when it is said that electricity will some day supersede steam, we can assert with a similar show of reason that flying may some day take the place of locomotion on the common road. At present there is no way of accomplishing the wished-for result. Steam produces electricity, and it is the most economical agent for producing it.

It is interesting to reflect that steam is produced by the combustion of a past vegetation. The great tree ferns of the carboniferous age required for their complex life the transformation of the sun's energy into chlorophyl and the tissues of these strange growths. This varied transformation is again made manifest in the wonderful action of coal-tar dyes in modern photography. Thus the original action of electro-magnetic waves shown in complex growths of vegetation, buried in the earth for ages, becomes evident again in a grand series of transformations. The burning of a fossilized tree trunk produces steam, steam is converted into motion, motion into electricity, and electricity into heat and light.

I have said that steam is at present our cheapest source of electricity. With the best engines it requires little more than a pound of coal to produce a horse power. This is a cheaper horse power than we can produce by water power, for the regulation and control of the supply of water is more difficult than that of a supply of steam. We thus see in New England the manufacturing towns of New Bedford and Fall River, where there is no water power, competing in number of spindles with Lowell and Lawrence, on the banks of the Merrimac River.

In studying the steps by means of which Faraday and Joseph Henry detected the small indications of a force which the steam engine has exalted into a mighty one, we are reminded of the slight rub which Aladdin gave to the lamp which was sufficient to summon a genius who could perform any tasks, from the most delicate one to the most tremendous. The feeble manifestation discovered by Faraday and Henry of what can be made a great force is called the force of magnetic induction. It may be said that it laid perdu with the possibility of being called into action by any one who possessed a coil of wire and a magnet. The reason that it remained undiscovered so long was due to the difficulty of obtaining well-insulated copper wire, and to the want of a sufficiently sensitive instrument to detect it.

The story of the discovery of magnetic induction by Faraday and Henry is most instructive, for it shows how an apparently slight and unimportant manifestation of energy can be exalted by proper means into a tremendous one. Faraday remarked, after detailing his experiments on magnetic induction: "The various experiments of this section prove, I think, most completely

the production of electricity from ordinary magnetism. That its intensity should be very feeble and quantity small can not be considered wonderful, when it is remembered that, like thermoelectricity, it is evolved entirely within the substance of metals retaining all their conducting power."

The steam engine has exalted this apparently feeble effect discovered by Faraday into a power which is only limited by that of the steam engine or the water power which we employ.