140 SIMPLER ORGANIC SUBSTANCES 



gaseous medium their surfaces were heated. In the course 

 of this heating the oxides of iron and the silicates were 

 reduced and the latter became gaseous. The gases escaped 

 and this increased the proportion of iron in the planetesimals. 

 The smallest ones were completely volatilised, the rather 

 larger ones were converted into alloys of iron and nickel 

 while the still larger ones only formed alloys of iron and 

 nickel on their surfaces, their interiors remaining at low 

 temperatures and retaining their original composition. At 

 this stage the ' proto-Earth ' lost a considerable part of its 

 mass. According to Kuiper, the mass of the Earth at present 

 is only 1/1,200 part of that of the original protoplanet. 



A considerable increase in the proportion of iron in the 

 Earth resulted fiom this loss of silicates and other volatile 

 substances. Some water managed to remain on the proto- 

 Earth in the form of hydrates of silicates and as condensed 

 water. Nitrogen was retained in the form of metallic nitrides 

 and salts of ammonia, e.g. ammonium chloride. The most 

 stable forms in which carbon was retained were carbides of 

 iron and graphite, for the primaeval hydrocarbons, methane 

 in particular, must have escaped from the zone in which the 

 Earth-like planets were being formed. Thus, at the end of 

 the third postulated (hot) stage in the formation of planets, 

 large amounts of hydrogen, helium, methane, water and 

 nitrogen disappeared from the proto-Earth and its further 

 development proceeded in the absence of any significant 

 quantities of gas. The temperature of all objects on the proto- 

 planet therefore fell very quickly by radiation. Thus the 

 Earth was evidently formed at comparatively low tempera- 

 tures approaching those of the present day. It was formed 

 somewhere near to the centre of gravity of the protoplanet 

 and included in itself all the bodies which moved around 

 it as satellites. 



In this way our planet was accumulated from the planetesi- 

 mals, which were iron and siliceous bodies similar to the 

 present-day meteorites. The iron nucleus of the Earth differ- 

 entiated itself from Tvhat was originally a nearly homogeneous 

 mass of iron and siliceous phases considerably later, in geo- 

 logical times. At the same early stage too, the Earth must 

 certainly have lost those gases, above all hydrogen, which its 



