At the same time there is a continuous flux of energy. Organic matter contains potential energy, the energy of chemical combination; and during its passage through the living being it is gradually stripped of this energy and returned to the mineral world. The first step in synthetic biology is the addition of potential energy to matter, the reduction of an oxide, the separation of a salt into its radicals, the production of some endothermic chemical combination. The energy stored up by such processes can be again liberated as heat, that fire which the ancients with wonderful prescience long ago recognized as the symbol of life. Attempts have been made to differentiate a living being by the nature of its chemical combinations, the so-called organic compounds. It was supposed that life alone could realize these and cause the production of the various substances which form the structure of living beings. Of late years, however, a large number of these organic substances have been artificially produced in the laboratory, and the synthetic problems which remain are of the same order as those which have been already solved. As one learns to know the mineral kingdom and the living world more intimately the differences between them disappear. Thus a living being was supposed to be characterized by its sensibility, _i.e._ its faculty of reaction against external impressions. But this reaction is a general phenomenon of nature; there is no action without reaction. Neither can the {149} reaction to internal impressions, immediate or deferred, be considered as the characteristic of life, since osmotic growths exhibit a most exquisite sensibility in this direction. Since, then, the faculty of reaction is a general property of matter, the characteristics of life in the lower organisms are only three in number, viz. nutrition, growth, and reproduction by fission or budding. But crystals are also nourished and grow in the water of crystallization. They have moreover a specific form, and every biologist who wishes to establish a parallel between the phenomena of the living and the mineral world is wont to compare living beings with crystals. Crystals, it is said, affect regular geometric forms, salient angles, and rectilinear edges, while living beings have rounded forms without any geometric regularity. Another supposed distinction is that living beings are nourished by intussusception, whereas crystals increase by apposition. Again, living beings are said to assimilate and transform the aliment they absorb, whereas crystals do not transform the matter which is added externally to their structure. Another supposed difference is that living things eliminate and discharge their products of combustion, while the evolution of a crystal is accompanied by no such elimination. Finally, the phenomenon of reproduction is said to be the exclusive characteristic of a living being; but crystals may also be