ii8 



SIMPLER ORGANIC SUBSTANCES 



Collisions between atoms of gas and particles of dust there- 

 fore lead to a cooling of the gas, making it colder in the 

 presence of dust than in the absence of it." 



The interstellar gas consists almost entirely of hydrogen 

 which is the most abundant element of the cosmos in general 

 (accounting for 90 per cent of its mass)/* The work of H. 

 Kramers and D. ter Haar^^ has shown that the simplest 

 hydrocarbon radicals, ch and ch+^ are formed in interstellar 

 space. However, H. C. Urey*" considers that, as a result of 

 the catalytic activity of the dust and the presence of large 

 amounts of hydrogen in the clouds of gas and dust, all free 

 radicals would be converted into stable molecules. He con- 

 siders it probable that methane is formed, although more 

 complicated hydrocarbon molecules may also occur. On the 

 basis of their own investigations D. R. Bates and L. Spitzer" 

 suggest that when a cloud of dust of the usual density moves 

 towards a hot star the temperature of the particles of dust 

 will rise and, at a particular distance from the star, the CH4 

 will evaporate and will later dissociate to give ch and ch+. 



Thus we may observe the same widespread formation of 

 hydrocarbons, both in the incandescent atmospheres of the 

 stars and in the cold clouds of gas and dust. There can be 

 no possible doubt that the hydrocarbons were formed abio- 

 genically in these situations. 



The position is the same within the narrower confines of 

 our own planetary system. Although it is difficult to study 

 the planets spectroscopically, a considerable number of facts 

 as to the chemical constitution of the atmospheres of the 

 planets has now been accumulated. As early as 1935 these 

 facts were brought together by H. N. Russell in his book 

 The solar system and its origin.^" The more recent discoveries 

 may be found in H. C. Urey's book The planets, their origin 

 and development, to which reference has already been made, 

 and also in the collection of papers edited by G. Kuiper and 

 published under the title The atmospheres of the Earth and 

 planets.^^ 



The planets of the solar system may be divided into two 

 groups according to their chemical composition: the group 

 of large planets, which includes Jupiter, Saturn, Uranus and 

 Neptune, and the group of planets resembling the Earth 



