'vital force,' an influence (sometimes regarded as external) which modifies the behavior of matter and energy without affecting their quantity. But as we can not exclude the probability of similar directing and modifying influences in the inorganic world, it is best provisionally to regard the causes of life as present alike in living and not-living substances—conspicuous in living substance because coordinated, but hardly observed in not-living substance, owing to incoordination. If a simile be needed, it may be found in the behavior of light: for certain properties of light, though ever present, become evident only when coordinated by polarization; and polarization, be it noted, is a purely physical action.
While every element present in living substance may assist in the work, the energy-traffic is carried on chiefly amongst the four elements, nitrogen, oxygen, carbon and hydrogen. Nitrogen is remarkable for the instability of its chemical compounds—the readiness with which they change their composition—and there is little doubt that on this property depends the extreme sensitiveness of living substance. Carbon and hydrogen have the property of combining together to build up complex compounds, with great storage of potential energy; whereas the same compounds during their oxidation expend their energy in the form of mechanical work, heat, etc. The energy traffic consists of two alternating phases: (1) the accumulation of energy, or 'anabolic' phase, which is always coincident with deoxidation and the formation of complex chemical compounds: and (2) the dispersion of energy, or 'catabolic' phase, coincident with oxidation of the complex substances, which are thereby converted into simpler substances, as carbonic acid and water. In these processes nitrogen is intimately concerned: it is believed to act as the carrier, taking up each element or group of elements and passing it on in a new state of combination.
All the energy of life is derived ultimately from the sun. A little of this comes indirectly through lightning, which in passing through the air forms ammonia and oxides of nitrogen. These, being carried by rain into the ground, are the constant source of nitrogen for vegetable, and indirectly for animal life. A much larger quantity of energy is well known to be taken direct from the sunshine by plants and used in their anabolic processes. This energy is appropriated by animals in their food; and whether in the vegetable or in the animal, it assists in many alternations of anabolism and catabolism before it is completely dispersed.
The range of temperature suited to terrestrial life is comparatively narrow. All vital actions are suspended temporarily, some permanently, if subjected to a temperature near the freezing point; while the highest that most organisms can bear lies somewhere between 35° and 45° Centigrade (95° and 113° Fahrenheit). Only the spores of certain bacteria can survive boiling. It is therefore probable that if