(5) As the polar caps melt, they are bordered by a blue belt, which retreats with them. This excludes the possibility of their being formed of carbon dioxide, and shows that of all the substances we know the material composing them must be water.
(6) In the case of the southern cap, the blue belt has widenings in it in places. These occur where the blue-green areas bordering upon the polar cap are largest.
(7) The extensive shrinkage of the polar snows shows their quantity to be inconsiderable, and points to scanty deposition due to dearth of water.
(8) The melting takes place locally after the same general order and method, Martian year after year, both in the south cap,
(9) And in the north one. This is evidenced by the recurrence of rifts in the same places annually in each. The water thus let loose can, therefore, be locally counted on.
(10) That the south polar cap is given to greater extremes than the north one, implies again, in view of the eccentricity of the orbit and the tilt of the axis, that deposition in both caps is light.
(11) The polar seas at the edges of the caps being temporary affairs, the water from them must be fresh.
(12) The melting of the caps on the one hand and their reforming on the other affirm the presence of water vapor in the Martian atmosphere, of whatever else that air consist.
(13) Since water vapor is present, of which the molecular weight is 18, it follows from the kinetic theory of gases that nitrogen, oxygen, and carbonic acid, of molecular weights 28, 32, and 38 respectively, are probably there, too, owing to being heavier.
(14) The limb-light bears testimony to this atmosphere.