The fact that all the organic compounds of the food may serve as a source of energy, and as the larger portion of the food is utilized for energy purposes, it seems wise to give this phase of nutrition a somewhat special consideration. The living animal, either as a whole or in some of its parts, is constantly in motion. This means that the animal mechanism is ceaselessly performing work. Even if the body is apparently quiet, the heart beats, pumping blood to all parts of the body, the lungs are expanded and contracted, and the stomach and intestines keep up the movements which are essential to digestion. Besides, a living body is the seat of continuous, invisible, and complex chemical and physical changes, such as the breaking up of compounds in digestion and their rebuilding in assimila-tion, that, if not work in the common meaning of the term, are its equivalent. Walking, pulling, lifting, pumping blood, breathing, masticating, digesting and assimilating food, represent, then, a great variety of operations of living machines.

Now work requires the expenditure of energy. The projection of a rifle ball through space at the rate of two thousand feet per second is work. The ball does not move of itself, but is propelled by the application of the energy stored in a powerful explosive. Back of every one of our great mechanical operations, such as pumping, grinding, and moving railroad trains, will always be found some sort of energy, and what is true of machinery made of wood and iron is equally true of that made of bone and muscle. The fact that the mechanism is alive does not abrogate a single physical law, so that the fundamental principles of energy as applied to machines are directly applicable to the activities of animal life.

It is safe to go farther, and say that the animal organism does not originate energy. Among the fundamental conceptions upon which all our knowledge of chemical and physical laws rests is this, that energy and matter are indestructible, and, moreover, that the sum total of these in the universe is unchangeable. If, then, man expends the muscular energy necessary to propel a bicycle over one hundred miles of road, the equivalent of this must have been supplied to his body from some outside source. He could not create it. We know that this is so, and we also know it must be conveyed to him in his food.

164. Manifestations Of Energy

In considering this subject it is natural to first ask, what is energy? This is a difficult question to answer in a popular way, and the physicists' definition would hardly serve our purpose. All we can do, perhaps, is to illustrate it by pointing out some of its manifestations. Let us resort to an old illustration. Every farmer's boy has doubtless seen a black-smith hamme7an iron rod until it was red-hot. The motion of the hammer-head descending with great velocity was suddenly arrested when it came in contact with the rod. This descent of the hammer-head illustrated one form of active energy, viz., motion of a mass of matter. When the hammer met the iron rod on the anvil, the mass motion ceased. Was the energy therefore lost? Not unless our fundamental conception is wrong, and we find that in this case it is not. The physicist teaches us that the energy represented by the moving mass of matter, that is, the hammer-head, was communicated to the molecules of the iron rod, and as the vibrations of the molecule increased in rapidity, the rod grew hotter and hotter. Here we have another illustration of energy, viz., the motion of the molecule or heat into which the energy of mass motion has been transformed. The iron rod might have been heated in another way, - by plunging it into burning charcoal, and here the heat energy would come from the combustion of the carbon. Somehow, when it is deposited in the plant, there becomes stored in this carbon, in a way about which we can only theorize, what perhaps we may call the chemical energy of the atom, which, when combustion occurs, is changed into heat or molecular motion.

Perhaps another illustration may still further serve our purpose. A small dynamo is being run by a pair of horses working in a tread power such as is used for threshing grain. The horses are constantly climbing up a moving treadway and thereby communicating muscle energy to motion of machinery. This motion is, by the dynamo, converted into electricity, which, by passing through the carbon film of an incandescent lamp and there meeting resistance, is in part, at least, transformed into heat. We have then, in a chain, muscular vibration, motion of the mass (pulleys, wheels, etc.), electricity and heat, all active energies and all transferable the one into the other. This is a fairly good picture of what goes on with the horse or man, externally and internally, in sustaining life and performing labor. From these phenomena we learn that not only are there several forms of energy, but that one form is transferable into another.