Page:Popular Science Monthly Volume 59.djvu/260

250 of lime separate from the water as a solid precipitate, the second part of the carbon dioxide would be free to pass from the water into the atmosphere. We are thus led to consider the agencies which have caused the deposition of lime from dilute solution.

Modern views of chemical combinations regard compounds in solution as going through a constant interchange of reactions, by which ions pass continuously from one association to another, as in the grand chain the dancers weave in and out with touch of hands. The dance of the ions, more technically called dissociation, is most active in dilute solutions, and is promoted by higher, retarded by lower temperatures. It has been shown by experiment that bicarbonate of lime may be dissociated by agitating the solution, and there are occurrences of calcareous formations which indicate that the monocarbonate is deposited as a result of such action. Thus it is probable that, through this process, warm seas surrender to the air a notable amount of carbon dioxide, but that the contribution becomes insignificant or ceases when the waters are chilled.

Under favorable conditions the ocean abounds in organisms which secrete normal carbonate of lime as parts of their structures. They swarm in the warm waters of tropical oceanic currents, they exist in multitudes on the warm shallows where the sea spreads over the margins of continental masses with a depth not exceeding 100 fathoms, but they are rare or are replaced by species without hard parts in cold waters. The physiological reactions by which these organisms obtain the normal carbonate from the water are not definitely known. They may take it from bicarbonate in solution, or by reaction on sulphate of lime setting free sulphuric acid, which attacks the bicarbonate. In any case, the effect is to fix one ion of carbon dioxide in the solid normal carbonate, and to free the second ion, which may pass into the air. The enormous volume of organic calcareous deposits now forming, and the massive limestone strata of past ages, largely or wholly of organic origin, attest the importance of the process. Life may be considered the most important of those agencies which restore carbon dioxide to the atmosphere, but it is narrowly conditioned by limitations of habitat and warmth.

Carbon dioxide absorbed in sea-water is yielded to the atmosphere and returned by it under varying conditions of tension of the gas, of barometric pressure, and of temperature. At moderate temperature the sea gives up the gas freely, and would supply a deficiency gradually brought about in the atmosphere. But colder waters hold it faster, and may even take carbon dioxide from an already depleted atmosphere.

Thus the processes of dissociation by chemical and organic agencies and of absorption depend upon temperature, and through this dependence promote the prevailing tendency of climatic changes. If the