PHYSICAL CONDITIONS OF THE SUN AND ITS RELATION TO THE EARTH'S ATMOSPHERE
Origin of the Sun and the Planets. --A modified form of Laplace's nebular hypothesis assumes that when the early nebula, which filled the space included, at least, within the orbit of the sun's remotest planet, and which was composed of dust, or gaseous particles, or of dust particles surrounded by gaseous envelopes, began to contract under the influence of its own gravity, it commenced a rotation about its common center. Contraction caused it to gain in temperature, and a reduction in the diameter of the rotating mass effected an increase in the velocity of rotation. This process continued until increasing centrifugal force just equaled the attraction of gravitation, when, with the minutest further gain in the velocity of rotation, the outer rim was abandoned--not thrown off. This rim continued to rotate for a time, as the rings of Saturn are still doing, when it broke up and gathered about a center of its own, but continued to pursue the same orbit as before its disruption. Thus was formed, according to the most generally accepted hypothesis, the first and outer planet of the solar system; and thus, it is fair to assume, was formed our moon from matter abandoned by the earth, and in similar manner were the satellites of all other planets fashioned. As the sun continued to contract through a long period of years--measured by tens of millions--it grew hotter, increased in velocity of rotation, and successively abandoned the matter that now forms the several planets and the numerous asteroids of its system.
Solar Heat: Past, Present, and Future. --Laplace's theory assumes that originally the early nebulæ were at a very high temperature and that the sun and its satellites were formed under conditions of great original heat. This part of his theory is not now generally accepted, as the conditions are better met by assuming that the heat was evolved as the result of the reduction of gaseous volume. The investigations of J. Homer Lane, of Washington, D. C., have assisted in giving one a clearer idea as to how