162 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
their satellites, all moving in the same direction round the nucleus of 
the nebula which became thesun. Kant’s hypothesis explained nearly 
all of the available observational data within the framework of physics 
as it was developed at the time. 
Later on, about 1800, the French mathematician, Laplace, inde- 
pendently proposed a modified form of the Kant hypothesis which, 
even though it was not given much weight by its author, soon became 
widely accepted as the concept upon which much of geology was 
founded. Laplace went further than Kant in explaining how the 
primordial nebula condensed into planets. He assumed that in the 
beginning the nebula was hot and spinning slowly, that the gas con- 
tracted as it cooled and therefore increased its spin in accordance with 
the law of conservation of angularmomentum. As the spin increased, 
he reasoned, rings of gas would be thrown off by centrifugal action and 
each ring would condense into a planet. It is now recognized that no 
such condensation of hot gas at the rim of a spinning nebula would take 
place, but Laplace’s speculation was important in that he introduced 
two new factors: the idea that the earth condensed from hot gases, and 
the consideration of angular momentum in the solar system. 
Not until 1895 was the Laplace hypothesis seriously challenged. 
By that date geology had come into its own as a science, and T.C. 
Chamberlin, an American geologist, considered the geological evi- 
dence incompatible with the concept of a hot gaseous sphere cooling to 
become the present earth. Instead, he proposed the planetesimal 
hypothesis, in which the earth and other planets were built by accretion 
of cold particles (the planetesimals) which were moving around the 
sun under its gravitational attraction. Together with an astronomer, 
F. R. Moulton, he suggested that such planetesimals might have 
resulted from a near-collision between another star and oursun. The 
planetesimal hypothesis introduced two new concepts: that the earth 
was built by accretion of cold solid material, and that another star 
was involved in forming the solar system. The near-collision pre- 
sumably being a rare event, this represented a return, in part, to the 
old concept of a catastrophic origin. 
During the last 50 years, most of the thinking on this problem has 
been divided between the two widely divergent hypotheses of Laplace 
and Chamberlin. Did the earth start hotter or colder than at present? 
Has it condensed and contracted, or grown by accretion? Was its 
origin a commonplace occurrence in a nebula (many of which can be 
seen in the sky), or due to a highly unusual near-collision between 
stars? Whatever drawbacks these incomplete speculations may have 
had, they have provided definite concepts on the basis of which further 
research has been and yet remains to be done. 
