104 G. F. Becker — Kant as a Natural Philosopher. 



The planets in agglomerating become fluid. Newton, and 

 indeed Descartes, had previously pointed out that planets had 

 passed through the fluid condition. Kant seemingly ascribed 

 the fusion of planets and satellites to the impact of their com- 

 ponent parts.* 



Kant expressed in the clearest manner and with sufficient 

 demonstration his view that the period of rotation of the moon 

 was reduced to coincidence with that of its revolution by tidal 

 action on the satellite while it was still liquid. He draws the 

 conclusion that the moon is somewhat younger than the earth. f 

 He also demonstrates that oceanic tides retard the rotation of 

 the earth, and predicts that the earth will eventually turn one 

 side only to the moon. 



In a cooling planet, according to Kant, there is a rough and 

 partial separation by density. Such a process is still progress- 

 ing in Jupiter which Kant regards as still fluid. The planets 

 eventually become solid throughout.^: The obliquity of the 

 axes of rotation to the planes of revolution may be in part due 

 to changes in shape accompanying consolidation and in part 

 due to irregular distribution of velocities in the gathering mass 

 of a planet. The progressively increasing density of the 

 planets as the sun is approached is due to the varying propor- 

 tion of the heavier elements in the contracting nebula; the 

 lighter elements being deflected into orbital paths more easily 

 than the heavier ones. Kant in 1755 regarded the sun as a 

 solid burning mass, attributiug its small density to the fact 



* Kant's Werke, vol. i, p. 302. 



f Ibid., p. 183 to 186. It may be of interest to translate a feu- sentences liter- 

 ally from this paper, ' : Ob die Krde in ihrer Umdrehung um die Acbse einige 

 Yeranderung erlitten habe." " The water of the ocean covers at least one-third 

 (sic) of its surface and is kept in continual motion by the attraction of the above 

 mentioned celestial bodies [the sun and moon]. Moreover ihis motion is in one 



direction, exactly opposed to the rotation Since this flow is opposed to the 



rotation of the earth, we have here a cause which may be counted upon to retard 

 and diminish rotation continuously to the extent of its capacity .... The termi- 

 nation of this change of rotation will occur when the earth's surface, from the 

 point of view of the moon, shall be relatively at rest, i. e. when it rotates in the 

 same time as the moon revolves. If it were fluid throughout, the moon's attrac- 

 tion would soon bring its rotation down to this fixed remainder. This immediately 

 reveals the cause which has compelled the moon in its journey round the earth 

 always to expose to it the same aspect. . . . The attraction which the earth exerts 

 on the moon, acting on the satellite while it was still fluid, must have reduced the 

 rotation of the moon (formerly no doubt greater than now) to this fixed residue 

 in the manner just explained." 



% This is distinctly stated in Allgemeine Naturgeschichte; and in Ob die Erde 

 veralte. One of the sentences in the preceding footnote is equivalent to an argu- 

 ment that, were the earth not solid, internal tidal friction would long since have 

 reduced its period of rotation to that of the moon's revolution. This ingenious 

 mode of reasoning on the earth's rigidity has not been revived or quantitatively 

 tested so far as I know. It is worthy of consideration. In the Physische Geo- 

 graphic, 1802, however, he asserts that while the surface irregularities of the 

 earth show solidity of the superficial portion, it does not follow that time enough 

 has yet elapsed to carry consolidation to the center. 



