Professor F.J. Allen considers that the most important element in protoplasm, and that which confers upon it its most essential properties in the living organism--its extreme mobility and transposibility--is nitrogen. This element, though inert in itself, readily enters into compounds when energy is supplied to it, the most striking illustration of which is the formation of ammonia, a compound of nitrogen and hydrogen, produced by electric discharges through the atmosphere. Ammonia, and certain oxides of nitrogen produced in the atmosphere in the same way, are the chief sources of the nitrogen assimilated by plants, and through them by animals; for although plants are continually in contact with the free nitrogen of the atmosphere, they are unable to absorb it. By their leaves they absorb oxygen and carbon-dioxide to build up their woody tissues, while by their roots they absorb water in which ammonia and oxides of nitrogen are dissolved, and from these they produce the protoplasm which builds up the whole substance of the animal world. The energy required to produce these nitrogen-compounds is given up by them when undergoing further changes, and thus the production of ammonia by electricity in the atmosphere, and its being carried by rain into the soil, constitute the first steps in that long series of operations which culminates in the production of the higher forms of life. But the remarkable transformations and combinations continually going on in every living body, which are, in fact, the essential conditions of its life, are themselves dependent on certain physical conditions which must be always present. Professor Allen remarks: 'The sensitiveness of nitrogen, its proneness to change its state of combination and energy, appear to depend on certain conditions of temperature, pressure, etc., which exist at the surface of this earth. Most vital phenomena occur between the temperature of freezing water and 104° F. If the general temperature of the earth's surface rose or fell 72° F. (a small amount relatively), the whole course of life would be changed, even perchance to extinction.'