220 LIGHT, VEGETATION AND CHLOROPHYLL 



costs IJd., the equivalent in coal ^d., in petrol Id., in sugar 

 4d., and in butter Is. Only human labour is rather highly 

 valued, since the equivalent of a kilowatt-hour costs more 

 than 30s. 1 



Energy obtained through chlorophyll is therefore far from 

 being appreciated at its just value or, more exactly, electrical 

 energy is still so scarce that it is glaringly over-estimated in 

 comparison. Yet in assessing the real advantages of con- 

 verting the kinetic energy of flowing water into electricity, it 

 is necessary to consider whether any better use could be made 

 of the water. 



The rate of solar radiation at the outside of the earth's 

 atmosphere is 2 calories per minute per square centimetre. 

 Under the best conditions, the soil receives only about half 

 of it. Taking into account the length of the day and the time 

 of insolation, we can estimate the number of calories received 

 annually per square metre as between 1-2x10^ and 1-5x10^ 

 calories. Now, a man's normal requirements, about 3,000,000 

 calories^ per day, are below this total. If all the calories 

 coming from the sun were transformed into food through the 

 agency of chlorophyll, a man could hve on 1 square metre. 



In the best conditions, a leaf fixes, in dry weight, more 

 than a hundredth of the calories received, but a good field 

 of maize transforms into grain only a two-hundredth part of 

 these calories. It therefore seems impossible to nourish a man 

 on less than one twentieth of an acre. We are still far from 

 doing that. 



There is no doubt that an effort can and must be made to 

 banish the bugbear of over-population that the neo- 

 Malthusians raise with such persistence. Of particular 

 importance are the quantity and quahty of our essential 



'^Translator's Note — The figures in this paragraph have been con- 

 verted into the average values prevailing in Great Britain in 1955. 



^Here, as elsewhere when we speak of calories without qualification, 

 we mean small calories. Biochemists, in discussing human needs, for 

 example, always quote them in kilo-calories without adding the quali- 

 fication which appears to them useless. We prefer to sacrifice custom to 

 clarity. 



