the elements which most concern us are those heading the respective columns.

From the foregoing analyses, which express the percentage composition, it will be seen that carbon is by far the most important constituent of coal. Carbon is a chemical element which is found in a crystalline form in nature as the diamond, and which forms a most important constituent of all living matter, whether animal or vegetable. Woody fibre contains a large quantity of this element, and the carbon of coal is thus accounted for; it was accumulated during the growth of the plants of the Carboniferous period.

Now carbon is one of those elementary substances which are said to be combustible, which means that if we heat it in atmospheric air it gives out heat and light, and gradually disappears, or, as we say, burns away. The heat which is given out during combustion represents the chemical energy stored up in the combustible, for combustion is in fact the chemical union of one substance with another with the development of heat and light. When carbon burns in air, therefore, a chemical combination takes place, the air supplying the other substance with which the carbon combines. That other substance is also an element—it is the invisible gas which chemists call oxygen, and which forms one-fifth of the bulk of atmospheric air, the remainder consisting of the gas nitrogen and small quantities of other gases with which we shall have more to do subsequently. When oxygen and carbon unite under the conditions described, the product is an invisible gas known as carbon dioxide, and it is because this gas is invisible that the carbon seems to disappear altogether on combustion. In reality, however, the carbon is not lost, for matter is as indestructible as energy, but it is converted into the dioxide which escapes as gas under ordinary circumstances. If, however, we burn a given weight of carbon with free access of air, and collect the product of combustion and weigh it, we shall find that the product weighs more than the carbon, by an amount which represents the weight of oxygen with which the element has combined. By careful experiment it would be found that one part by weight of carbon would give three and two-third parts by weight of carbon dioxide. If, moreover, we could measure the quantity of heat given out by the complete combustion of one pound of carbon, it would be found that this quantity would raise 14,544 lbs. of water through 1° F., a quantity of heat corresponding to over eleven million foot-pounds of work, or about seven and three-quarters horse-power per hour.

Here then is the chief source of the energy of coal—the carbon of the plants which lived on this earth long ages ago has lain buried in the earth, and when we ignite a coal fire this carbon combines with atmospheric oxygen, and restores some of the energy that was stored up at that remote period. But the whole of the energy dormant in coal is not due to the carbon, for this fuel contains another combustible element, hydrogen, which is also a gas when in the free state, and which is one of the constituents of water, the other constituent being oxygen. In fact, there is more latent energy in hydrogen, weight for weight, than there is in carbon, for one pound of hydrogen on complete combustion would give enough heat to raise 62,032 lbs. of water through 1° F. Hydrogen in burning combines with oxygen to form water, so that the products of the complete combustion of coal are carbon dioxide and water. The amount of heat contributed by the hydrogen of coal is, however, comparatively insignificant, because there is only a small percentage of this element present, and we thus come to the conclusion that nearly all the work that is done by our steam-engines of the present time is drawn from the latent energy of the carbon of the fossilized vegetation of the Carboniferous period.

The conclusion to which we have now been led leaves us with the question as to the origin of the energy of coal still unanswered. We shall have to go a step further before this part of our story is complete, and we must form some kind of idea of the way in which a plant grows. Carbon being the chief source of energy in coal, we may for the present confine ourselves to this element, of which woody fibre contains about 50 per cent. Consider the enormous gain in weight during the growth of a plant; compare the acorn, weighing a few grains, with the oak, weighing many tons, which arises from it after centuries of growth. If matter is indestructible, and never comes into existence spontaneously, where does all this carbon come from? It is a matter of common knowledge that the carbon of plants is supplied by the atmosphere in the form of carbon dioxide—the gas which has already been referred to as resulting from the combustion of carbon. This gas exists in the atmosphere in small quantity—about four volumes in 10,000 volumes of air; but insignificant as this may appear, it is all important for the life of plants, since it is from this source that they derive their carbon. The origin of the carbon dioxide, which is present as a normal constituent of the atmosphere, does not directly concern us at present, but it is important to bear in mind that this gas is one of the products of the respiration of animals, so that the animal kingdom is one of the sources of plant carbon.

The transition from carbon dioxide to woody fibre is brought about in the plant by a series of chemical processes, and through the formation of a number of intermediate products in a manner which is not yet thoroughly understood; but since carbon dioxide consists of carbon and oxygen, and since plants feed upon carbon dioxide, appropriating the carbon and giving off the oxygen as a waste product, it is certain that work of some kind must be performed. This is evident, because it has been explained that when carbon combines with oxygen a great deal of heat is given out, and as this heat is the equivalent of the energy stored in the carbon, it follows from the doctrine of the Conservation of Energy, that in order to separate the carbon from the oxygen again, just the same amount of energy must be supplied as is evolved during the combustion of the carbon. If a pound of carbon in burning to carbon dioxide gives out heat equivalent to eleven million foot-pounds of work, we must apply the same amount of work to the carbon dioxide produced to separate it into its constituents. Neither a plant nor any living thing can create energy any more than it can create matter, and just as the matter composing a living organism is assimilated from external sources, so must we look to an external source for the energy which enables the plant to do this large amount of chemical work.

The separation of carbon from oxygen in the plant is effected by means of energy supplied by the sun. The great white hot globe which is the centre of our system, and round which this earth and the planets are moving, is a reservoir from which there is constantly pouring forth into space a prodigious quantity of energy. It must be remembered that the sun is more than a million times greater in bulk than our earth. It has been calculated by Sir William Thomson that every square foot of the sun’s surface is radiating energy equivalent to 7000 horse-power in work. On a clear summer day the earth receives from the sun in our latitude energy equal to about 1450 horse-power per acre. To keep up this supply by the combustion of coal, we should have to burn for every square foot of the sun’s surface between three and four pounds per second. A small fraction of this solar energy reaches our earth in the form of radiant heat and light, and it is the latter which enables the plant to perform