The scientific meaning of energy is the capacity of doing work; and work may be defined as the movement of weight. Thus, a unit of work is reckoned as a weight of one pound raised through a space of one foot.

Energy

There are several forms of energy (heat, motion, electricity, and attraction, for instance), all of which can mutually reproduce each other. We may divide attraction into gravitation, cohesion and chemical affinity. Gravitation is the power of attraction which all bodies in the universe have for each other. Cohesion and chemical affinity act only when the bodies (molecules or atoms) which are influenced by them, are very close together. Electricity, as we all know, can generate motion, light and heat. Heat by means of the steam engine, for example, can produce motion and electricity. Motion produces heat when we rub the palms of our hands briskly together; electricity and heat, when we rub a stick of sealing wax on a silk handkerchief; and light and heat, when, by using a flint and steel, the small particles of stone which are broken off, violently impinge against each other, and thus produce sparks. The principle here involved is termed the correlation of forces, and means, as Grove expresses it, "that any force capable of producing another may in its turn be produced by it"

Energy exists either as active energy, which is energy at work, or passive energy (potential energy), which is energy at rest. If we wind up a clock without the works being put into movement, the active energy in the form of motion which we transferred from our muscles to the machinery, will remain as passive energy until the pendulum begins to swing. Then, as long as the works keep going, their motion will become converted into heat. If we throw a stone straight up into the air, the active energy possessed by the stone on quitting our hand, will gradually become converted into passive energy by the force of gravitation, until the highest point of its trajectory is reached, at which moment it will be at rest. During its descent, it will gradually lose its accumulated passive energy, until on striking the ground, its active energy of motion, due to gravitation, will become converted into heat.

A similar case of the conversion of' attraction into heat is seen, when water comes in contact with quicklime, and slaked-lime is produced. Some bodies which have a strong chemical affinity for each other, require an impetus to effect their union. Thus, the carbon in the materials (wood, coal, charcoal, or peat) for a fire, will not combine with the oxygen of the air without being set alight. Here we have a case somewhat analogous to that of a stone which, instead of being thrown up in the air and coming down again, sticks in the foliage of a lofty tree during its ascent. It would then require an impetus or shove-off, in order to convert its potential energy into active energy.

We have seen that by pouring water on quicklime, chemical affinity is converted into heat; but if we apply heat to slaked-lime, we shall have a portion of the heat changed into chemical affinity by the water being driven off. A similar action takes place in a lime kiln during the conversion of limestone (carbonate of lime) into quicklime.

When the water in a kettle is boiled over a fire under ordinary conditions, neither the water nor the steam rises above boiling point (212° F.); because the surplus heat is occupied in keeping the particles of steam separate from each other. While accomplishing that work, heat is said to be latent (inappreciable by the senses). If, however, we check the expansion of the vapour by means of pressure, as in a digester, the heat which is unutilised for keeping the particles of steam apart, will tend to raise the temperature of both the water and the steam above 212° F. But the moment the artificial pressure is removed, the surplus heat will become converted into latent heat, with the result that the steam will expand, and its temperature and that of the water will fall to boiling point. The cooling effect which the evaporation of perspiration has on the skin, is due to the conversion into latent heat of a portion of the heat that was present in the skin. The raising of water from the surface of the earth in forming clouds is an everyday instance of work done by latent heat.

The food of those plants which are not parasites consists of carbonic acid (CO2) water (H10), mineral salts (such as phosphates, nitrates and sulphates of lime, potash and magnesia), and small quantities of oxygen and ammonia. Tubercles on the roots of plants appear to exert a strong influence in facilitating the absorption of nitrogen either simple or combined. By means of the energy received in the form of heat and light from the sun, plants are able to carry on their vital functions and to store up substances which are possessed of energy in the form of chemical affinity for oxygen. In the manufacture of these substances (e.g. woody fibre, starch, oil, and vegetable albumen) by a plant, the oxygen-containing compounds which are absorbed by the plant (carbonic acid and water, for example) are deprived more or less of their oxygen, which the plant gives off into the atmosphere. Thus, in order to manufacture 10 parts by weight of woody fibre, the plant requires 16.3 parts of carbonic acid gas and 5.5 parts of water, the loss being 11.8 parts of oxygen. Hence, when woody fibre is burnt, its potential (stored-up) energy, in the form of chemical affinity for oxygen, is productive of a considerable amount of heat. As starch has the same chemical composition as woody fibre, it yields to the animal which eats and digests it the same amount of energy as woody fibre produces in a fire; for in both we have the act of combustion, which is oxidation. The chemical reaction in both cases is expressed as follows: -

6 CO2 + 5 H1O = C6H10O5 + 12O

(Carbonic (Water.) (Starch or (Oxygen.) acid.) woody fibre.)

In vegetable oils and fats the proportion of oxygen is still less, and consequently they possess more stored-up energy.

In the animal body, the oxygen which is required for the combustion of food is obtained, as we have seen, from the air breathed in by the lungs.

The nitrogenous compounds formed in plants by the agency of the rays of the sun, also possess energy in the form of chemical affinity for oxygen, though to a much less extent than fat or starch, as we may learn from their composition (approximatively, 53.5 parts carbon, 7 parts hydrogen, 23.3 parts oxygen, and 16.5 parts nitrogen), and from the fact that chemical affinity is stored up in urea and hippuric acid, in which products almost all the nitrogen contained in the plant food of the horse leaves his body. The life of plants which are eaten by horses (non-parasitic plants) is a process of deoxidation; that of animals, one of oxidation.

The absorption of carbonic acid gas in plants is effected by chlorophyll, which is the green colouring matter of plants and which is formed only in sunlight. Hence, plants which have no chlorophyll, are unable to obtain their carbon from the carbonic acid of the air, and are consequently obliged to feed on other plants or animals. Fungi (mushrooms, toadstools, etc.) and bacteria (the disease germs of glanders and tuberculosis, for instance) are varieties of such parasitic plants.