C. The mental element, under the direction of which certain new processes were inaugurated, and certain previous processes were modified and controlled.
A. The Chemical Work of Life.
The peculiar chemical phenomena connected with life chiefly concern the carbon compounds. In the inorganic world the carbon compounds are few and simple. In the organic world they become extremely numerous and complicated. These compounds are very unstable, for the greater part, and their partial decomposition gives rise to many additional compounds. Some of the true organic compounds and some of their decomposition products have the power of combining with inorganic substances, and so produce an additional series of semi-organic combinations. The total number of the compounds thus directly and indirectly connected with life greatly exceeds that of all inorganic compounds. Their mass, however, is very greatly inferior.
Life material chiefly atmospheric.—In the building up of the organic compounds, a necessary step is the decomposition of certain inorganic compounds. The chief of these is the carbon dioxide of the atmosphere and hydrosphere, the decomposition of which furnishes the carbon needed for the organic compounds. On this account carbon dioxide may be regarded as in some sense the basal material or the fundamental food of the organic kingdom, and hence it plays a radical rôle in the life-history of the earth.
Water, and the constituents of water, oxygen and hydrogen, play a larger part quantitatively, but a less distinctive part.
Nitrogen is also an essential element, and usually stands next to carbon, oxygen, and hydrogen in quantity.
These, it will be noted, are all atmospheric constituents, and the material of life is, therefore, dominantly atmospheric. This is even true of aquatic life, for it lives largely on the atmospheric constituents dissolved in the water. The function of life, considered from the material point of view, is not only fundamentally concerned with the atmosphere, and intimately dependent on its conditions, but its most important material effects appear to lie in its modification of the constitution of the atmosphere.
The non-atmospheric factors.—The atmospheric constituents are not, however, the only elements intimately connected with the life function. Compounds of sulphur, phosphorus, potassium, sodium, chlorine, iron, calcium, magnesium, silicon, and other elements are more or less essential to the life of many organisms, or are employed by them for their skeletons, coverings, etc. Incidentally, nearly all the common elements become intimately related to living organisms either in the relations of active elements in their physiological functions, or of passive elements in their structure or in their auxiliary parts.
Three Classes of Effects.
Out of life processes grow three rather distinct classes of results: (1) changes in the amounts and proportions of the constituents of the atmosphere and, to some slight extent, of the hydrosphere and lithosphere; (2) aid or hindrance to inorganic processes, such as disintegration, erosion, and deposition; and (3) distinctive products, either (a) of organic matter that would not have come into the existing combination but for life, such as peat, lignite, amber, etc., or (b) of special forms of inorganic matter that would not have arisen but for life, such as coral deposits, shell-marl, diatom ooze, etc.