Further, the manifoldness of chemical energy is the cause of the peculiar manner in which it is transformed into other forms. In the other forms of energy the transformation can be effected by the body itself. Nothing else is required. If a stone is thrown and it hits against a wall, it loses its kinetic energy, the greater part of which changes into heat. But in order to liberate the chemical energy of, say, coal, the coal alone is not sufficient; another chemical substance is required, the oxygen of the air. The interaction of the two substances produces a new substance, and it is only during this process that a corresponding part of the chemical energy is liberated. There are a few chemical processes also (allotropic and isomeric changes) in which a single substance without the co-agency of another substance can give off energy. But the quantity of energy thus obtained is infinitely small as compared to that liberated by the interaction of two or more substances. Because of the necessity of two or more substances to co-operate in giving off chemical energy, the opportunity for the transformation of chemical energy is less than for the transformation of the other forms of energy, and this is the main reason why it can be conserved so long and so easily. All that is necessary is to prevent contact with another substance. This is a problem, it is true, which from the point of view of strict theoretical rigor it is almost impossible to solve. In practice, however, it can be easily solved for periods of time long enough at least to require special means to enable us to recognize that it is only a temporary and not a fundamental solution. Scientifically expressed, the cause of this is that the diffusion of the various substances in one another can theoretically never be completely eliminated, while on the other hand the velocity of the diffusion over distances measured only by decimeters is extremely low.
[PART IV]
THE BIOLOGIC SCIENCES
54. Life.
Among the bodies in our environment that are ponderable and have mass the animate beings are so strikingly distinguished from the inanimate that in most cases we have not the slightest doubt whether a body belongs to the one kind or to the other, even if in some cases we happen not to be familiar with its peculiar form. In the first place, therefore, we must answer the question in a general way and tell what the distinguishing peculiarities are that mark them off one from the other.
The first peculiarity is this, that living organisms are not stable but stationary forms. This distinction is based upon the fact that a stable form is at rest or unchangeable in all its parts, while a stationary body, though it seems unchangeable in its form, internally undergoes a constant change of its parts. Thus, a brass faucet is a stable body, since it not only preserves its form and function permanently, but consists at all times of the same material and shows the same peculiarities, such as stains, defects in form, etc. It cannot be said, it is true, that it will remain completely unchanged for all time. Its metal suffers a gradual chemical and mechanical deterioration. But this is not essential to the existence of the faucet, since the deterioration varies greatly with circumstances, and if conditions are ideal it can be reduced to zero.
On the other hand, the jet of water flowing from the faucet is a stationary body. In favorable circumstances it can assume a constant form, so that at a hasty glance it might be taken for a stable glass rod. On closer examination it will be found that the parts of water of which it is formed are not the same at any given instant as the instant before, each part that has flowed away being replaced by another just as large following it.
From this difference in the nature of the two bodies results a difference in their behavior. If I make a mark on the faucet with a file, the mark remains permanent. But even if I sever the entire water jet with a knife, the cut is healed the next moment, because by reason of the continuous flow of the water, the severed place is instantly eliminated from the body. Owing to this nature peculiar to stationary bodies, they have the capacity of being healed or of regeneration.
For a body to continue permanently in a stationary condition the material of which it is composed must be permanently supplied. If we turn off the faucet, the water jet immediately disappears or "dies." Evidently, therefore, a stationary body can subsist by its own means only if it has the property or capacity to provide itself continually with the necessary material. This material consists in the main of ponderable or chemical substances of definite physical and chemical properties, and thus the change of substance, metabolism, appears as a necessary property of the stationary body. In order, however, that metabolism should take place we must have free energy, or energy having the capacity to work, since it is only free energy that can cause substances to change, just as every phenomenon in the world implies the equalization of free energy. For a stationary body to exist independently, therefore, it must have the property of being able spontaneously to possess itself of the necessary substances and of free energy. But since, as we have already said, the energy of organisms is stored up and used in the main in the form of chemical energy, the two tasks which a stationary body has to perform, that of meeting the need for substances and for energy, are as a rule externally combined. In organisms these two necessities combined are called nutrition, and thus we recognize in the capacity for self-acquisition of nutrition another essential property of organisms.