SOURCES OF ENERGY 



i8r. 



It is a peculiarity of ultraviolet radiation that its activity 

 is very selective. Sometimes it affects only a very limited 

 part of some particular molecule. Very delicate and specific 

 alterations may therefore be brought about by the action of 

 ultraviolet radiation on substances whose specific absorptive 

 capacity is strictly limited to a particular part of the ultra- 

 violet spectrum. An example of this, which is well known 

 to biologists, is the conversion of ergosterol to vitamin Do by 

 ultraviolet irradiation. ^^^ 



C9H17 



In this reaction the complicated molecule remains unchanged 

 as a whole. It is only in the second ring of the phenanthrene 

 nucleus that one bond is broken, with the formation of a 

 double bond in the side chain. Other very diverse but always 

 highly specific stereoisomeric transformations of organic mole- 

 cules are well kno^vn to occur on irradiation with ultraviolet 

 light of strictly defined ^vavelength."^ In particular we must 

 note the cis-trans isomerisation of very many organic com- 

 pounds, both simple^^*^ and considerably more complicated 

 in structure."^ Finally, if ultraviolet light is circularly polar- 

 ised it can affect the optical isomerism of the compounds 

 formed, thus creating the conditions for direct asymmetric 

 synthesis. (We shall deal with this subject in more detail 

 somewhat later.) 



Taking into account all that has been discussed, ^ve may 

 assume that in the atmosphere of the primaeval Earth many 

 diverse and complicated organic substances ^vere formed from 

 comparatively simple ones, mainly methane, ammonia, water 

 vapour and hydrogen sulphide, under the influence of elec- 

 tric discharges and ultraviolet radiation. With rain and other 

 precipitations these complicated substances fell into the 

 primitive hydrosphere. Having fallen into this new medium 

 they continued to change and become even more compli- 



