University of Virginia Library

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 nebuæ. 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.

In 1869 Sir William Thomson, afterwards Lord Kelvin, commented that Kant's

attempt to account for the constitution and mechanical origin of the universe on Newtonian principles only wanted the knowledge of thermodynamics, which the subsequent experiments of Davy, Rumford and Joule supplied, to lead to thoroughly definite explanation of all that is known regarding the present actions and temperatures of the Earth and of the Sun and all other heavenly bodies.

These are, apparently, the enthusiastic comments resulting from the re-discovery of Kant's papers. A present-day writer would not speak so decisively of them, but we must all bow in acknowledgment of Kant's remarkable contributions to our subject, published when he was but 31 years old.