Physical exercise plays a most important part in regard to our need for food, in that it greatly increases this need apart from the question of body weight. Other causes of increase, such as cold, male sex, have a comparatively unimportant influence. The influence of brainwork is, if any, so inconsiderable that it can scarcely be calculated.

Our food consists of all the substances of which our bodies are formed, i.e., protein {and the so-called albuminoids gluten and gelatin), fat, carbohydrates (= sugar and starch), water, and those salts of which sodium, calcium, potassium, iron, carbon, phosphorus, and chlorine are essential parts. We take in all these substances by the mouth, and by "food" we at once and exclusively think of these. But as "food "we may also reckon oxygen which we inspire, and which is contained in most of the substances in the body and forms a necessary means of metabolism and of the production of energy.

If one wishes to reckon as foods all the substances which may be consumed by the organism and which give off heat, one must include alcohol. Every gram of this gives off seven calorics when completely oxidised to C02 and H20. But alcohol is not only an abnormal constituent of the body, but also a poison which can only be taken in very small quantities without obvious harmful effects; let us say only 1/4 grm. in twenty-four hours per kilo of body weight. Its effect upon mechanical and other power of work is doubtful; for my part I incline to the opinion that in the long run it lowers power of work.

Protein has been called the plastic food-substance, as it forms the most important solid substance in the protoplasm of the cells; while carbohydrates and fats are considered as "respiratory" or heat (and movement) producing foods. However, it may now be considered as certain (see below) that protein also under certain conditions may be concerned in the production of vital force. This is also the case with gluten, which can certainly not take the place of protein, but is of considerable value in saving protein. An animal receiving only gluten, fat, carbohydrate, water, and salt lives considerably longer on this food than it does if gluten is omitted.

Salts and water have no value for combustion, and are thereby distinguished from the real food substances. But both are absolutely necessary constituents of the tissues and indispensable for the physical and chemical processes of life. As regards salts especially, their exclusion from our diet causes death more quickly (through poisoning by organic sulphates) than the simultaneous exclusion of protein, fat, and carbohydrate.

In our food substances, carbon, hydrogen, oxygen and nitrogen atoms, as regards protein and carbon, hydrogen and oxygen atoms, as regards fat and carbohydrates, are combined with one another in such a way as to form molecules of the respective substances, so that these represent chemically stored force. The work necessary for this is originated by the light of the sun, and performed by means of the chlorophyll (green colouring matter) of plants or modifications of this. The force lies in the fact that the atoms in the protein, fat and carbohydrate, with their comparatively loose chemical combination, are similarly built up with each other to form (very complicated) molecules. By oxidation in the organism these very complicated molecules (formed only by loose chemical combination) break up into very much simpler, more firmly combined molecules. When, therefore, the strong attraction between the atoms (attempt towards union) which constitutes chemical energy is done away with by their union, the latter is changed to heat. Many people now think that the whole quantity of energy first becomes heat, and that a portion of it afterwards is in some unknown manner changed to movement.

We measure mass movement in kilogrammetres and heat in calories. A kilogrammetre is the amount of work necessary to lift one kilo one metre. A (large) calorie is the amount of heat necessary to raise the temperature of a litre of water (volume reckoned at 20° C.) 1° C. A calorie corresponds, according to L. Hermann, to 426 kilogrammetres of work or force; 1 kilogrammetre is therefore = 0.0002347 calories. Others count a calorie = 425 kilogrammetres.

In connection with terms of work and allied terms I would refresh the reader's knowledge of physics by reminding him that a kilogrammetre is a definite term of work - that it requires a definite amount of work to raise a kilo 1 metre, and that the work thus stored up as elastic force expresses itself as a definite amount of vital force when the said weight again falls 1 metre. As the unit for vital force - -kilogrammetre - is the same as for work, so work = vital force. By combining the term of work with a condition of time we obtain quite another idea, which we call power of work or effect. What is usually called horse-power is that effect or power of work which enables a machine to perform 75 kilogrammetrcs of work in a second. As a measure of vital force we take the work which a body can perform by its velocity. If the mass of the body = m, its velocity = v, the vital force = mv2 / 2. For other fundamental equations in dynamics see the text-books on physics.

Since the Congress in Paris in 1881, electricians have adopted an absolute system with a unit of length = 1 cm. or 0.01 of the length of the standard metre in Paris, a unit of mass = 1 grm. or 0001 of the standard kilogram in Paris, a unit of time = 1 second or 1/60 of 1/60 of 1/24 of an average twenty-four hours. Further, as unit of velocity they have taken a distance of 1 cm. per second; as unit of acceleration an increase of this velocity by the same unit of velocity; as unit of force 1 dyn = the force which gives to the mass of 1 grm. an acceleration = the said unit of acceleration; as unit of work 1 erg = the work performed by 1 dyn when the distance in the direction of the force is 1 cm. (1 joule = 10,000,000 ergs). The unit of power of work or effect = a power of work of 1 erg in 1 second (a watt = an effect of 1 joule in 1 second).