What is it that determines whether or not an energy-transformation will occur? It is the condition that a difference of the intensity-factors of the energy in different parts of a system exists. Water will flow from a higher to a lower level, doing work as it flows, if it is directed through a motor. Electricity will flow if there is a difference of electrical potential. A chemical reaction will occur if two substances before interacting possess greater chemical potential than do the products which may possibly be formed during the interaction. Coal and oxygen possess greater chemical potential than do carbon dioxide and water, therefore they will combine, forming carbon dioxide and water. Energy-transformations will therefore occur wherever it is possible that differences of intensity or potential can become abolished. The energy that may thus flow from a condition of high to a condition of low potential, undergoing a transformation as it flows, is the available energy of the system of bodies in which it is contained. A closed vessel surrounded by an envelope impervious to heat, and containing a mixture of oxygen and hydrogen, is an isolated system containing available energy. Let the mixture be fired by an electric spark, and heat is evolved. The total energy of the system is unaltered in amount, but the available energy has disappeared, since the heated water vapour is incapable of undergoing further transformations while it forms part of its isolated system.[12] [12] It is really necessary to lay stress on the distinction between available and unavailable energy, as it is one which many biologists appear to ignore. Thus, a popular book on the making of the earth attempts to argue that essential distinctions between living and inorganic matter are non-existent. One of these distinctions is that organisms absorb energy, and this author points to the absorption of “latent heat” by melting ice as an example of the absorption of energy in a purely physical process. Consider a system consisting of a block of ice and a small steam boiler. We can obtain work from this by the melting of the ice--that is, its “absorption of latent heat.” The system, ice at 0° C. + steam at 100° C., possesses available energy, but the system, melted ice + condensed steam, both at the same temperature, contains none. The molecules of water at 0° C. “absorb energy,” that is to say, their kinetic energy becomes greater, but their available energy in the system has disappeared. In saying that the organism absorbs energy, we mean, of course, that it accumulates available energy, that is, the power of producing physical transformations. (See further, appendix, p. 366.)