between these convertible forms is fixed and invariable. From a given quantity of chemical energy represented, let us say, by a certain weight of coal, we can get a certain fixed amount of heat and no more. We can employ that heat to work a steam-engine, which we can in turn use as a source of electricity by causing it to drive a dynamo-machine. Then this doctrine of science teaches us that our given weight of coal in burning evolves a quantity of heat which is the equivalent of the chemical energy which it contains, and that this quantity of heat has also its equivalent in mechanical work or in electricity. This great principle—known as the Conservation of Energy—has been gradually established by the joint labours of many philosophers from the time of Newton downwards, and foremost among these must be ranked the late James Prescott Joule, who was the first to measure accurately the exact amount of work corresponding to a given quantity of heat.

In measuring heat (as distinguished from temperature) it is customary to take as a unit the quantity necessary to raise a given weight of water from one specified temperature to another. In measuring work, it is customary to take as a unit the amount necessary to raise a certain weight at a specified place to a certain height against the force of gravity at that place. Joule’s unit of heat is the quantity necessary to raise one pound of water from 60° to 61° F., and his unit of work is the foot-pound, i.e. the quantity necessary to raise a weight of one pound to a height of one foot. Now the quantitative relationship between heat and work measured by Joule is expressed by saying that the mechanical equivalent of heat is about 772 foot-pounds, which means that the quantity of heat that would raise one pound of water 1° F. would, if converted into work, be capable of raising a one-pound weight to a height of 772 feet, or a weight of 772 lbs. to a height of one foot.

This mechanical equivalent ought to tell us exactly how much power is obtainable from a certain weight of coal if we measure the quantity of heat given out when it is completely burnt. Thus an average Lancashire coal is said to have a calorific power of 13,890, which means that 1 lb. of such coal on complete combustion would raise 13,890 lbs. of water through a temperature of 1° F., if we could collect all the heat generated and apply it to this purpose. But if we express this quantity of heat in its mechanical equivalent, and suppose that we could get the corresponding quantity of work out of our pound of coal, we should be grievously mistaken. For in the first place, we could not collect all the heat given out, because a great deal is communicated to the products of combustion by which it is absorbed, and locked up in a form that renders it incapable of measurement by our thermometers. In the next place, if we make an allowance for the quantity of heat which thus disappears, even then the corrected calorific power converted into its mechanical equivalent would not express the quantity of work practically obtainable from the coal.

In the most perfectly constructed engine the whole amount of heat generated by the combustion of the coal is not available for heating the boiler—a certain quantity is lost by radiation, by heating the material of the furnace, &c., by being carried away by the products of combustion and in other ways. Moreover, some of the coal escapes combustion by being allowed to go away as smoke, or by remaining as cinders. Then again, in the engine itself a good deal of heat is lost through various channels, and much of the working power is frittered away through friction, which reconverts the mechanical power into its equivalent in heat, only this heat is not available for further work, and is thus lost so far as the efficiency of the engine is concerned. These sources of loss are for the most part unavoidable, and are incidental to the necessary imperfections of our mechanism. But even with the most perfectly conceivable constructed engine it has been proved that we can only expect one-sixth of the total energy of the fuel to appear in the form of work, and in a very good steam-engine of the present time we only realize in the form of useful work about one-tenth of the whole quantity of energy contained in the coal. Although steam power is one of the most useful agencies that science has placed at the disposal of man, it is not generally recognized by the uninitiated how wasteful we are of Nature’s resources. One of the greatest problems of applied science yet to be solved is the conversion of the energy latent in coal or other fuel into a quantity of useful work approximating to the mechanical equivalent much more closely than has hitherto been accomplished.

But although we only get this small fraction of the whole working capability out of coal, the actual amount of energy dormant in this substance cannot but strike us as being prodigious. It has already been said that a pound of coal on complete combustion gives out 13,890 heat units. This quantity of heat corresponds to over 10,000,000 foot-pounds of work. A horse-power may be considered as corresponding to 550 foot-pounds of work per second, or 1,980,000 foot-pounds per hour. Thus our pound of coal contains a store of energy which, if capable of being completely converted into work without loss, would in one hour do the work of about five and a half horses. The strangest tales of necromancy can hardly be so startling as these sober figures when introduced for the first time to those unaccustomed to consider the stupendous powers of Nature.

If energy is indestructible, we have a right to inquire in the next place from whence the coal has derived this enormous store. A consideration of the origin of coal, and of its chemical composition, will enable this question to be answered. The origin of coal has already been discussed. Chemically considered, it consists chiefly of carbon together with smaller quantities of hydrogen, oxygen, and nitrogen, and a certain amount of mineral matter which is left as ash when the coal is burnt. The following average analyses of different varieties will give an idea of its chemical composition:—

There are in addition to these constituents small quantities of sulphur and a certain variable amount of water (5 to 10 per cent.) in all coals, but