SOURCES OF ENERGY 163 



ultraviolet irradiation to give formaldehyde which then 

 underwent polymerisation. 



E. Rabinowitch has reviewed the extensive, though highly 

 contradictory, literature on the subject of the formation of 

 formaldehyde from aqueous solutions of carbon dioxide 

 during ultraviolet irradiation. From this literature it appears 

 that such formation, if it occurs at all, does so only to a very 

 limited and sometimes scarcely perceptible extent. 



Under natural conditions this reaction could not give rise 

 to large amounts of organic substances, as the oxygen formed 

 in it would very soon set up an ozone screen, preventing the 

 access of short-wave ultraviolet radiations to the louver lavers 

 of the atmosphere. This is also the usual explanation for the 

 absence of reactions by which co, is reduced under the in- 

 fluence of ultraviolet radiation on the Earth at present. 



N. R. Dhar and A. Ram,''* however, claim to have found 

 some thousandths of i per cent of formaldehyde in rainwater. 

 They suggest that this formaldehyde was formed photochemi- 

 cally in that part of the atmosphere which lies oiUside the 

 ozone screen. It would, however, be hard to prove that these 

 infinitesimal amounts of formaldehyde were formed in this, 

 rather than in some other way. 



The second source of energy in the atmosphere of the 

 Earth is electrical discharges, either silent or in thunder. 

 It is very hard to calculate the amount of this energy. If, as 

 is usually done, we assume that under contemporary condi- 

 tions one flash of lightning strikes the ground for every square 

 kilometre of the surface of the Earth each year,^^ and that 

 the mean energy of a flash is lo^'' ergs,"**^ then the ^vhole 

 surface of the Earth receives 5-1 x 10* x 10^^ = 5-1 x 10" 

 ergs/year or 1-2 x 10^^ kcal/year. It follows that the energy 

 of electrical discharges is several orders lower than that of 

 ultraviolet light. This calculation, however, only takes into 

 account the noisy discharges of thimderstorms and it may be 

 that the energy of silent discharges in the atmosphere is also 

 quite considerable. There is also reason to suppose that 

 thunderstorms were more frequent in primaeval times. 



As early as 1899 F. Allen''^ suggested the possibility that 

 the energy of electrical discharges in the atmosphere might 

 have been used in carrying out many organic syntheses on 



