The Formation of the Earth 69 



the later fate of which must, on account of their coUision with one another, have 

 been far more involved and complicated than that indicated by Kuiper. 



A more important criticism of Kuiper's and Fesenkov's hypotheses for us 

 to-day is based, not on astronomical, but on physical and chemical considera- 

 tions. Data as to the chemical composition of the Earth and meteorites contradict 

 the idea that the Earth was formed from a hot, massive protoplanet [9-17]. We 

 must emphasize that the contradiction only becomes manifest when one considers 

 all the mutual impHcations of the chemical data. There is some chemical evidence 

 which does not contradict the idea that the Earth was formed by such a process 

 as Kuiper and Fesenkov postulate. This is the part of the evidence which has 

 been surveyed in the works of, for instance, Eucken and Latimer, who therefore 

 arrived at a wrong conclusion [12]. Furthermore, I. S. Shklovskii [18, 19] had 

 shown that the process of thermal dissipation of gaseous aggregates takes place 

 extremely slowly. It would be impossible, within the period allowed by cosmo- 

 gonie theory, for the protoplanet to have got rid, by this means, of the tremendous 

 amount of hydrogen which supposedly entered into its composition. 



Thus, Kuiper's hypothesis is open to most serious objections of an astronomical, 

 physical and chemical nature. Objections of almost the same kind could be made 

 to the views of V. G. Fesenkov. The replacement of the idea of the separation 

 of hot gaseous bodies from the Sun, by the idea of their formation from a cold 

 cloud of gas and dust was imsuccessful. 



I am glad to notice that A. P. Vinogradov who, some years ago, subscribed to 

 the view of the formation of the Earth from hot material, now supports the idea 

 that it was formed from cold material, as may be seen from his paper (p. 23). 



2. Inasmuch as I am taking part in the working out of O. Shmidt's 

 theory, it seems to me that this is the theory which gives the clearest and most 

 logical explanation of the essential features of the planetary system, both astro- 

 nomical and chemical. It must, however, be mentioned that Urey's theory, to 

 which I shall refer later, agrees, in many respects, with that of Shmidt. This 

 agreement, which is increasing as both the theories are further developed, is 

 very significant for two reasons. In the first place, Urey developed his theory 

 independently of Shmidt. In the second, Urey was, primarily, basing himself 

 only on chemical data and only secondarily on astronomical evidence, while 

 the development of Shmidt's theory began (1943-44) with the explanation of the 

 fundamental astronomical characteristics of the planetary system and only later 

 (1949-50) embraced the chemical aspects. It must be added that Urey, being a 

 chemist, made a detailed and penetrating analysis of the chemical evidence. 



The process of evolution of the protoplanetary cloud and the formation of 

 the planets as envisaged by Shmidt's theory, may be conventionally divided into 

 two stages. The first comprises the formation of numerous bodies with dia- 

 meters of several hundreds of kilometres (we shall call these bodies of an 

 asteroidal type or simply asteroidal bodies). The dust component of the cloud 

 must have settled comparatively quickly into its central plane, i.e. it became 

 collected into a very flat disc. The breakdown of this disc into separate aggregates 

 owing to the onset of gravitational instabihty would seem to have been the chief 

 way in which the asteroidal bodies were formed. If the properties of the dust 



