TRANSFORMATION OF HYDROCARBONS 157 



may be taken to be about 2-8 x lo^"* tons. According to 

 calculations of E. Rabinowitch,^ the entire vegetation of the 

 globe produces by photosynthesis i-2 x lo" tons of oxygen in 

 the course of one year. It follows that the entire amount of free 

 oxygen in the atmosphere could be produced by vegetation 

 in roughly 2,000 years — a period which is completely insigni- 

 ficant in relation to the thousands of millions of years during 

 which the Earth has existed. As early as 1856 C. Koene*^ put 

 forward the theory that the entire oxygen of the atmosphere 

 owes its origin to photosynthesis by green plants. This 

 idea was supported by many later authorities. It was, how- 

 ever, handled in a specially detailed way by V. Vernadskii." 

 Basing his arguments on a whole series of geochemical facts, 

 Vernadskii demonstrated the biogenic origin of the oxygen 

 in the present-day atmosphere. 



There is also, in the scientific literature, considerable dis- 

 cussion of the possibility of formation of molecular oxygen 

 by an inorganic mechanism. In particular, G. Tammann* 

 and, later, R. Wildt^ pointed out that a certain amount of 

 oxygen might have been formed by thermal dissociation of 

 water. This theory was not, however, sufficiently soimdly 

 based, and has met with serious opposition from the majority 

 of geologists and chemists. In any case, such oxygen as might 

 have been formed in this fashion would immediately have 

 been absorbed by mineral formations which were unsatur- 

 ated in respect of this element. 



There is much more in favour of the view that water 

 undergoes photolysis in the uppermost layers of the 

 atmosphere under the influence of ultraviolet radiation. S. 

 Arrhenius" discussed this possibility, and it has since been 

 considered by V. M. Goldschmidt,^^ W. Groth and H. Suess,^' 

 J. H. J. Poole,^' N. R. Dhar^-* and others. 



According to G. Rathenau,^^ water vapour absorbs in the 

 ultraviolet at wavelengths 1,780 A, 1,540 A and 1,340 A 

 (according to R. Mecke,^^ 1.390 A). As early as 1910 A. 

 Coehn^^ described the direct photochemical decomposition 

 of water into hydrogen and oxygen on ultraviolet irradiation 

 of water vapour. The equation is : 



light 



H2O + H2O > 2H2 -I- Oo 



