It must be remembered that as new shores are formed, or old ones are extended, the zones are kept within their bounds, and that as one zone creeps in on the land, those to the seaward move up also; so that where there was once a between-tide zone there may now be deep water. This change in the position of zones is very important; for certain animals and plants of the shore only live in certain zones, and their increase or decrease in numbers depends upon the corresponding state of their special locality. III.--THE ATMOSPHERE THE OCEAN OF AIR --AGNES GIBERNE Our earth has many robes. Closely-fitting garments come first, of brown soil or gray rock and green grass, with wide liquid underskirts of deep blue filling up the spaces between. Outside these are coverings more wonderful still; fragile, yet strong, transparent, almost invisible, folded around layer upon layer, or, as one might say, veil upon veil, each more gossamer-like than the last. These form earth’s surrounding atmosphere--a substance pervading everything, found everywhere. One may travel from the equator to the poles, one may journey by sea or by land, one may soar high in a balloon or descend deep into a mine, but one can never in this world go to a place where the atmosphere is not. A substance--for air can be felt; air has weight; air occupies space; air, like any other body, can be made hot or cold; air is composed of particles of substantial matter. Air has a faint bluish tint, which on a sunshiny day becomes in the sky a very pure and deep blue. This tint is not believed to be the natural color of the atmosphere. Were it so, the air would merely act the part of a blue pane of glass, rendering the white light of the sun blue as it reaches our eyes; but the blue of the atmosphere is known to be a reflected blue. If reflected, there must be something in the atmosphere to reflect it; and such indeed is the case. Perfectly pure air would doubtless be without color, but perfectly pure air we do not find. The whole atmosphere is full of multitudinous minute specks, so small as to be in themselves invisible, so light as to remain aloft. To the presence of these the blue tint is believed to be due. They scatter the light of the sun, and produce the blue effect. A beam of strong white light, caused to pass through a liquid which contains a large supply of minute floating particles, is affected by them in a like manner. The short blue waves are more abundantly reflected than the long red waves; and so the water seems to be blue. This explanation serves for the deep-blue color of the ocean, as well as for the blue of the atmosphere.