APPENDIX 337 



of which made Ingenhousz famous.^ Ingenhousz showed that 

 the dissociation of carbonic acid inside the plant takes place 

 exclusively in the sunhght. In Priestley's later experiments 

 the plants probably did not get enough sunUght, and therefore 

 did not dissociate carbonic acid. 



Sunshine, the rays of the sun, are the very energy which 

 loosens and separates the particles of carbon from those of oxygen 

 during the decomposition of carbonic acid. Such an expression 

 as ' sunshine is a source of energy' may sound strange at first. We 

 know from daily experience how pleasant it is to warm oneself 

 in the sun, and yet a long chain of arguments and calculations is 

 necessary to persuade us that this is not only a source of energy, 

 but even a very considerable source of energy — moreover, that 

 this is almost the only source of energy used by man. In fact, 

 apart from the energy of the tide, used in some parts of Europe, 

 and which depends on the attraction of the moon (and also of 

 the sun), all the other motive forces, all other sources of energy 

 directly or indirectly depend upon the energy of sunhght. 

 The flow of water in rivers and the circulation of air in the 

 atmosphere, which set in motion our mills, are due to the sun. 

 The latent energy of fuel, as we have already seen and shall 

 presently see in greater detail, proceeds from the sun. Even 

 phenomena so remote as those of electricity, which we use for 

 practical purposes, can be connected with the activity of the 

 sun. It is the sun that shines in a Voltaic arc obtained 

 by means of a galvanic battery, which is easy to prove. The 

 electric current which makes the carbon white-hot is the result 

 of the oxidation in the batteries of a certain amount of metallic 

 zinc. This zinc, however, is not found in Nature as a metal ; 

 it is generally found in combination with oxygen, i.e. entirely 

 burnt down. In order to deoxidise it, to restore its capacity 

 for combustion, oxygen must be removed from it. This is done 

 by means of coal, which combines with the oxygen of the zinc 

 ore, and burns down to carbonic acid. But this coal, be it pit 

 coal or wood charcoal, has been derived in the plant from carbonic 

 acid, dissociated by sunlight. Thus it is that the rays of the sun 



1 Such is the current opinion, but lately, after a careful study of the 

 question, I have come to the conclusion that Ingenhousz' priority is 

 more than doubtful, and that the fact of the dependence of this process 

 on sunlight was discovered by Priestley. 



Y 



