Since the time of Newton and Leibnitz the laws of ordinary dynamics have been settled; but between dynamics, and the so-called ‘imponderable forces,’ by which it was assumed that an explanation was furnished of heat, chemical action, electricity, magnetism and the rest, there was a great gulf. That has been bridged over by a common measure of value, giving rise to a general dynamic law of all the forces of the universe. This is known as the ‘Conservation of Energy.’ It demonstrates, with more or less completeness, that all physical phenomena are the different appearances resulting from different groupings of matter by force. Both matter and force are eternally changeless in quantity. They can be neither diminished nor increased, neither created nor destroyed. The ‘modes of motion’ that produce the varied phenomena are interchangeable. Light may become heat, heat may become electricity; but there is no loss. When motion seems to disappear, it is only transferred into another mode. The phenomena of light, heat, electricity, result from changes wrought in matter by motion. Every ray of sunshine, for instance, is an interwoven group of powerful energies. Whence do they come? From changes in the matter of the sun. For every measure of sunshine received by the earth an equivalent measure of solar matter has been changed. Through the sunbeam the varied forms of energy produced in the sun have been received, and stored up on this earth. For incalculable centuries the world has been clothed with abundant vegetation. This will only grow in sunlight. The energies in the sunbeam act on living matter, as on a spore or a seed, causing it to grow. That is to say, under the influence of vital action, the forces of the sunbeam are changed into wood. A section from a pine mast, or an oak trunk, stands for a measurable quantity of sunshine. But coal is wood; forests buried and carbonized. Coal then has, as it were, fixed the energies of the sunbeams that poured upon the earth millions of years ago. Put coal where it will be in contact with oxygen and heat, and what follows? You get heat, light, chemical action; that is to say, the locked-up energies of the sunbeams of a measureless past are set free.