122 SIMPLER ORGANIC SUBSTANCES 



smaller in size. This planet is assumed to have been formed 

 somewhere between the orbits of Mars and Jupiter. Its radius 

 is estimated at 2,500 - 3,000 km. and its mean density at 

 3-8 (S. Orlov,'* V. Fesenkov," A. Zavaritskii^^ and others). 

 R. A. Daly" even tried to build a model of this hypothetical 

 planet, analogous to the meteoritic model of the Earth, 

 having a core of iron and nickel enclosed in a geosphere of 

 silicates and basalt. 



On the other hand O. Shmidt, B. Levin,'" and other 

 workers deny the possibility that meteorites were formed by 

 the disintegration of a ' mother ' planet, because they con- 

 sider such a disintegration physically inexplicable. They see 

 meteorites as splinter bodies like asteroids, formed at remote 

 stages of the evolution of the protoplanetary cloud, formed, 

 perhaps, in the same region as the Earth and therefore 

 having a similar over-all chemical composition. 



Whichever hypothesis one supports, it is quite clear that 

 the study of the composition and structure of meteorites can 

 give a great deal of information relevant to the problem of 

 what were the primary compounds which appeared during 

 the formation of the Earth. 



All meteorites are commonly allocated to two basic groups, 

 stony and iron. An intermediate group is sometimes recog- 

 nised, the iron-stony meteorites." 



The iron meteorites are composed of so-called nickel iron, 

 which contains more than 90 per cent of iron, 8 per cent of 

 nickel, about 05 per cent of cobalt and small amounts of 

 phosphorus, sulphur, copper and chromium. In the stony 

 meteorites, which fall far more frequently on the Earth, the 

 percentage of iron is considerably lower. In these, silicates 

 and oxides of such metals as magnesium, aluminium, calcium, 

 sodium, etc., predominate. The discovery of 9 per cent of 

 constitutive water by A. Zavaritskii and L. Kvasha*" in the 

 Staroe Boriskino meteorite is of great interest. 



Carbon has been found in meteorites whenever it has been 

 looked for. The amount present is sometimes as low as some 

 hundredths of 1 per cent but some so-called carbon meteor- 

 ites contain up to 2 or even 45 per cent of carbon. 



As regards the isotopic composition of the carbon of 

 meteorites, the mean value of the ratio of ^^c to "c is 2 per 



