Whether the Wolf-Rayet stars have in general been formed from planetary nebulae is a different question: some of them certainly have. Wright has recently shown that the stellar nuclei of planetary nebulae are Wolf-Rayet stars, and he has formulated several steps in the process whereby the nebulosity in a planetary eventually condenses into the central star. The distribution of the planetaries and the Wolf-Rayet stars on the sphere affords further evidence of a connection. We saw. that the novae are nearly all in the Milky Way. The irregular, ring, planetary and stellar nebulae, plotted in Fig. 27, prefer the Milky Way, but not so markedly. The Wolf-Rayets, without exception, are located in the Milky Way and in the Magellanic Clouds, and those in the Milky Way are remarkably near to its central plane. 107 of these objects are known, 1 is in the Lesser Magellanic Cloud, and 21 are in the Greater Magellanic Cloud. The remaining 85 average less than 2 3/4 degrees from the central plane of the Milky Way.

We are obliged to say that the places of the novae, of the planetary and stellar nebulae, and of the Wolf-Rayets in the evolutionary process are not certainly known. If the Wolf-Rayet stars have developed from the planetaries, the planetaries from the novae, and the novae have resulted from the close approach or collision of two stars, or from the rushing of a dark or faint star through a resisting medium, then the novae, planetaries and Wolf-Rayets belong to a new and second generation: they were born under exceptional conditions. The velocities of the planetary nebulae seem to be an insuperable difficulty in the way of placing them between the irregular nebulae and the helium stars. The average radial velocity of 47 planetary nebulae is about 45 km. per second; and, if the motions of the planetaries are somewhat at random, their average velocities in space are twice as great, or 90 km. per second. This is fully seven times the average velocity of the helium stars, and the helium stars in general, therefore, could not have come from planetary nebulae. The radial velocities of only three Wolf-Rayet stars have been observed, and this number is too small to have statistical value, but the average for the three is several times as high as the average for the helium stars. We can not say, I think, that the velocities of any novae are certainly known.

If the planetaries have been formed from novae, especially the novae which encountered the fiercest resistance, the high velocities are in a sense not surprising, for those stars which travel with abnormally high speeds are the ones whose chances for collisions with resisting media are best; and, further, the higher the speeds of collision the more violent the disturbance. This line of argument also leads to the conclusion that the novae, planetaries and Wolf-Rayets belong not in general before the helium stars, but to another generation of stars. They may, and I think will, develop into a small class of helium stars having special characteristics; for example, high velocities.

KANT'S HYPOTHESIS

Immanuel Kant's writings, published principally in 1755, are in many ways the most remarkable contributions to the literature of stellar evolution yet made. Curiously, Kant's papers have not been read by the text-book makers, except in a few cases. We have already referred to his ideas on the Milky Way and on comets. In his hypothesis of the origin of the solar system, he laid emphasis upon the facts that the six known planets revolve around the Sun from west to east, nearly in the same plane and nearly in the plane of the Sun's equator; that the then four known moons of Jupiter, the five known moons of Saturn, and our moon revolve around these planets from west to east, and nearly in the same general plane; and that the Sun, our moon and the planets, so far as known, rotate in the same direction. These facts, he said, indicate indisputably a common origin for all the members of the solar system. He expressed the belief that the materials now composing the solar system were originally scattered widely throughout the system, and in an elemental state. This was a half century before Herschel's extensive observations of nebuae. Kant thought of this elemental matter as cold, endowed with gravitational power, and endowed necessarily with some repulsive power, such as exists in gases. He started his solar system from materials at rest. Most of the matter, he said, drifted to the center to form the Sun. He believed that nuclei or centers of attraction formed here and there throughout the chaotic structure, and that in the course of ages these centers grew by accretion of surrounding matter into the present planets and their satellites; and that in some manner motion in one direction prevailed throughout the whole system. Kant's explanation of the origin of the ROTATION of the solar system is unsound and worthless. We now know that such a cloud of matter, free from rotation, could not of itself generate rotation; it must get the start from outside forces. Kant's false reasoning was due in part to the fact that some of our most important dynamical laws were not yet discovered, in part to his faulty comprehension of certain dynamical principles already known, and probably in part to the unsatisfactory state of chemical knowledge existing at that date. This was half a century before Dalton's atomic theory of matter was proposed.

Kant asserted that the processes of combination of surrounding cold materials would generate heat, and, therefore, that the resulting planetary masses would assume the liquid form; that Jupiter and Saturn are now in the liquid state; and that all the planets will ultimately become cold and solid. This is in fair agreement with present-day opinion as to the planets, save that modern astronomers go further in holding that the outer strata of Jupiter and Saturn, likewise of Uranus and Neptune, down to a great depth, must still be gaseous. In 1785, after the principle of heat liberation attending the compression of a gas had been announced, Kant supplemented his statement of 1755 as to the origin of the Sun's heat. He attributed this to gravitational action of the Sun upon its own matter, causing it to contract in size: he said the quantity of heat generated in a given time would be a function of the Sun's volumes at the beginning and at the ending of that period of time. This is substantially the principle which Helmholtz rediscovered and announced in 1854, and which is now universally accepted—with the reservation of the past ten years, that radioactive substances in the Sun may be an additional factor in the problem.

Kant's paper of 1754 enunciated the theory that the Moon always turns the same face to the Earth because of tidal retardation of the Moon's rotation by the Earth's gravitational attraction; and that our Earth tides produced by the Moon will slow down the Earth's rotation until the Earth will finally turn one hemisphere constantly to the Moon. This principle was in part reannounced by Laplace a half century later, and likewise investigated by Helmholtz in 1854, before Kant's work was recognized.

Kant's speculations on a possible destruction and re-birth of the solar system, on the nature of Saturn's ring, and on the nature of the zodiacal light are similar in several regards to present-day beliefs.

Kant wrote:

'I seek to evolve the present state of the universe from the simplest condition of nature by means of mechanical laws alone.'