Page:Popular Science Monthly Volume 48.djvu/550

508 from below. Vertical exchanges of temperature also take place between higher and lower layers of the atmosphere, while the difference of temperature between the polar regions and the equatorial zone results in the assimilative movements of the general atmospheric circulation. As ocean currents, like the Gulf Stream of the Atlantic and the Kuro Siwo of the Pacific, carry heat from the warmer to the colder regions of the earth, so air currents arise out of those differences of heat and of barometrical pressure which it is obviously their function, as far as possible, to remove. All atmospheric movements, in fact, however local or general their character may be, are either movements of direct assimilation by which the atmosphere is seeking, so to speak, to bring all its areas into like temperature and pressure with each other, or are disturbances involved and arising indirectly out of such acts of assimilation. It is only because the work done by these movements is being constantly undone through the agency of influences, permanent and temporary, that differentiate areas of the atmosphere in every part of the globe—setting up, for example, unlikenesses of temperature and pressure between the equatorial and polar regions, between continents or islands and the surrounding oceans, or between any area of the earth's surface abnormally heated or cooled and the surrounding parts of that surface, as well as between seasonal variations in such inequalities—that we have cyclones and anticyclones, tornadoes, blizzards, land and sea breezes, mountain and valley winds, sand spouts and dust whirlwinds, as well as various periodical and more or less local disturbances all over the world. It should be added that meteorological phenomena do but illustrate the wider interchanges that take place in the ether system, since the constant distribution, as electro-magnetic disturbances, of movement differentially accumulated in material aggregates—whether such disturbances take place within purely local limits, as in circuits artificially set up, or on a universal scale, as by diffusion from solar bodies—are all cases of the distribution of movement, and therefore cases of assimilation.

The diffusion of molecules through each other is also a common form of assimilation. Gases, if brought together, permeate each other until a tolerably like constitution for every larger or smaller area of the total volume has been reached; gas distributes itself equably through liquids, as in the case of effervescing drinks; solutions of salts brought into contact gradually intermingle. A soluble solid, when introduced into a liquid, usually assumes the liquid state to the extent of the capacity of the fluid for taking it up, as in the familiar case of sugar in tea or alum in water, while the liquid itself undergoes modification by the equable distribution of the particles absorbed. The uniform hardness of "hard" water, due to the presence of bicarbonate