relationships to planetary systems. They seem to be related. in their origin to New Stars and these in turn are thought to be produced by stars sweeping through clouds of meteoric or gaseous matter and attaining temporarily, from the swift impacts, an enormous brilliancy. The impact is so superficial, however, that the extreme brilliance is usually lost in a few days or weeks, and the star subsides through a stage like a planetary nebula into a peculiar type of star known as the Wolf-Rayet stars. The origin of the true planetary nebulæ has not, however, been observed, as they appear to possess a longer life than those which have originated in the past few centuries from new stars.

The ring nebulæ are few and special, having the form of a vortex ring.

The stellar nebulæ form another small group which look in the telescope like hazy stars.

By far the greatest number of the nebulæ are classified as spiral nebulæ, more than 120,000 of which have been made known by photography in connection with the greater telescopes. Their actual number must of course be far greater. These objects, unlike the other forms of nebulæ, avoid the Milky Way, and are scattered over regions where the stars are more widely spaced. They are very remote and may be entirely beyond the stellar system. This implies enormous magnitude. It seems probable that in general they possess high internal velocities, which implies in turn enormous masses to generate such velocities. These nebulæ possess spectra similar to those of stars rather than, like the other types of nebulæ, spectra of diffuse clouds of gas. Some astronomers look upon them, therefore, as possibly systems of stars rather than true nebulæ ; systems so remote as to give the appearance of faint cloud-like spirals, even when viewed under the highest powers of the telescope. A typical spiral nebula is shown in Plate I, B.

HYPOTHESIS OF PLANETS Derived FROM A PRIMAL NEBULA

The original hypothesis of Kant and Laplace. In 1754 and 1755 Immanuel Kant, the philosopher, published the most remarkable papers which had appeared up to that time upon the evolution of the solar system. He conceived matter to have been originally diffused and cold. From a position of rest it began to converge under the influence of gravitation and gave rise to the sun. In some manner he held that the matter in converging acquired a movement of rotation. Certain nuclei grew up independently from the center and gave rise to the planets and satellites. In 1785 he developed the idea that the contraction of the sun's mass would develop its heat, a view elaborated by Helmholtz in 1854 and generally held by astronomers at the present time. Thus Kant sought, and with a large measure of success, to evolve the present state of the universe from the simplest condition by means of mechanical laws alone.

In 1796 Laplace, one of the most eminent of French astronomers, published a general work on astronomy, and in a short note at the end of the appendix proposed a theory of the origin of the solar system which shortly became widely known as the nebular hypothesis. He was evidently unaware of Kant's work published forty-one years previously. Laplace is most noted for his mathematical work on celestial mechanics, yet he did not develop his hypothesis along such lines and apparently did not attach much importance to it. Nevertheless, it became the dominant idea in cosmic evolution throughout the next century.

Laplace postulated an original nebula as a very hot, gaseous mass extending beyond the orbit of the farthest planet and possessing a uniform rotation throughout, as if it were a solid body. Its size was the result of a balance between expansion from its heat and contraction from its gravitation. As it lost

heat it contracted and, with the same energy of rotation that it possessed before, necessarily revolved on its axis in a shorter time. At last a stage was reached where, in the equatorial belt, centrifugal force balanced gravitation and the matter subjected to this balance of forces could sink in no further. It is thought to have existed as a ring, left behind by the condensing mass. The ring, however, was unstable; it broke up and gathered into one body. During the further shrinking of the main mass other rings were in turn abandoned. Each gathered into a subordinate nebula, passed through an independent evolution, and the whole gave rise to the system of planets and their satellites.

Modifications of the nebular hypothesis. During the first half of the nineteenth century the nebular hypothesis was accepted by astronomers almost without question, but during the second half many serious dynamical objections were developed and a process of modification began, until now not much remains of the original conception of Laplace. A rather full statement of the hypothesis and the objections to it has been given recently by Campbell. A briefer summary and a citation of but a few of the modifications in the general concept must here suffice.

George Darwin, Lockyer, Faye, Fouché, Poincaré, and others have taken part in this work, and in the opinion of these mathematicians and astronomers the framework of the resulting structure is still sound, though subject of course to further modification as knowledge increases. It was shown that the original nebula need not have been hot, but, as perceived by Kant, would develop heat from its self-condensation. A loose swarm of cold meteorites would suffice as well as an original gas for the initial state. The mass could never have revolved as a unit body, as if it were a solid. On the contrary, the inner

2 Campbell, W. W., "The Evolution of the Stars and the Formation of the Earth." Scientific Monthly, vol. 1, 1915, pp. 189-194.

parts would be condensed and revolve fast while the outer parts were still diffuse and revolved slowly. The mode suggested by Laplace for the separation of the rings is also dynamically very unsatisfactory. Moulton has shown that the growth of the planets and the development of rotation in the same direction as their orbital motion could be much better attained from an initial state in which the component particles revolved in the same plane but independently and in highly elliptic orbits about the central nucleus. This is a wide departure from the idea of a circular ring revolving as a unit body.

Still more fundamental objections, emphasized by Chamberlin and Moulton, are found in certain of the existing dynamical relations of the solar system. It would be expected that in condensation the central mass would continually abandon matter from its equatorial zone, the inner planets would presumably have possessed the greater masses, and the final sun would now show a high speed of rotation, giving an equatorial diameter far greater than the polar. Such expectations are contrary to the facts. The sun revolves so slowly on its axis, once in twenty-five days, that it has no measurable equatorial bulge. In other words, centrifugal force is negligible in the sun. Furthermore, the equatorial plane of the sun, instead of lying precisely in the mean plane of the planets' orbits, is inclined seven degrees to such a mean plane.

A hypothesis to gain scientific credence must emerge successful from the test of observed facts and mathematical theory. The nebular hypothesis has not done so. It is on the defensive and has lost standing during the past generation. Nevertheless, it would be premature to abandon it entirely. It has the advantage of simplicity in that satellites, planets, and sun are explained as the products of a single process, convergence in a rotating nebula. But nature is often found to be complex in her operations, so that this advantage is of doubtful weight.