ORIGIN OF EARTH I33 



bodies but was scattered throughout the whole of the space 

 now occupied by the solar system. Under the influence of 

 gravitational forces the main mass of this material became 

 aggregated to form a large central body, the Sun. The rest 

 of the material took the form of a cloud of particles moving 

 round this body. Their paths crossed one another at all 

 angles. However, oAving to the reactions bet^veen the par- 

 ticles, their courses became more and more regular until, 

 finally, there emerged a flat s^varm of particles revolving 

 around the Sim, in nearly circular orbits. They approached 

 one another and joined together to form the ' germs ' of 

 planets. As these ' germs ' gre^v^ larger they began to attract 

 particles from more and more distant parts of the swarm 

 and as this went on the speed of their growth increased 

 gi'eatly and the ' germs ' turned into planets revolving around 

 the Sun in circular orbits in the same plane and direction. 



This so-called nebular theory of the origin of the solar 

 system was, at one time, pushed into the background by the 

 ' catastrophic ' hypothesis but came back into currency in 

 Western Europe and America after the appearance of the 

 works of C. F. von Weizsacker,"^ D. ter Haar^^^ and S. 

 Chandrasekhar"'' and in the U.S.S.R. in connection with 

 the studies of O. Shmidt."^ 



It is now the ruling hypothesis among cosmogonists, though 

 it is founded on completely new scientific facts. 



In Kant's time nothing Avas known about the nature of 

 the particles forming the planetary cloud nor about the way 

 in which they interacted. Astronomers now have at their 

 disposal very firmly based factual data concerning the chemi- 

 cal composition of the gases and dust particles which are 

 collected together in vast clouds in a number of parts of our 

 galaxy, and also concerning the temperature which prevails 

 in these clouds, the velocity and size of the particles, the 

 concentrations of the gas and dust in the various clouds, etc. 

 Modern theories of cosmogony make use of all these facts, 

 draw widelv on contemporary physics and chemistry and 

 apply the principles of thermodynamics and statistical 

 physics. This makes them more definite and enables them 

 to give a quantitative description of the phenomena which 

 are presumed to have occurred. At the same time the 



