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. LAPLACE'S HYPOTHESIS