LIGHT AND VEGETATION 35 



For this mass to be in equilibrium and to continue to 

 radiate as it does, it is necessary, according to the calculations 

 of physicists, that a central source of energy should maintain 

 its interior at a temperature of about 40 milUon degrees 

 Centigrade. This source of energy, which is nowhere near 

 exhaustion, can be derived only from the transmutation of 

 elements, the less stable being transformed into more stable 

 ones, as hydrogen into helium and then into heavier elements. 



The surface tends to cool by radiation and the heat lost 

 in this way is compensated by a supply of heat from the 

 interior. 



As almost the whole of the energy available to man — 

 solar heat, coal, petroleum, waterfalls, products of the earth, 

 etc. — has been created at the expense of solar energy, we can 

 say that we live by the transmutation of elements which takes 

 place at the centre of the sun. Our globe receives an infini- 

 tesimal part of the enormous energy thus Hberated. The trans- 

 mutations are not chimerical; physicists know how to produce 

 them in the laboratory, but their methods are very different 

 from those by which the alchemists hoped to make gold. 

 Extremely powerful apparatus has to be built, and, until 

 uranium was successfully disintegrated to make atomic bombs, 

 it was possible to produce only minute quantities of matter 

 which could not be weighed by the most deUcate micro- 

 balances. 



At the temperature existing at the centre of the sun such 

 transmutations take place naturally and matter is in a state 

 indescribable in ordinary language and inconceivable to our 

 imagination. Physicists can, however, calculate partially 

 what happens there. 



It is very easy to find the quantity of energy hberated, for 

 it finishes by escaping in the form of radiation, which only 

 needs to be measured. We can deduce what it must have been 

 when it left the sun by measuring it on the ground and 

 studying the properties of the atmosphere through which it 

 has passed. Research stations, such as that at Montezuma in 

 Chile, have been set up specially for this purpose. 



