336 THE LIFE OF THE PLANT 



panied not by liberation, or development of energy, but on the 

 contrary by its absorption and expenditure. While combina- 

 tion, i.e. combustion, takes place spontaneously, decomposi- 

 tion requires the participation of an extraneous force. In order 

 to bum down a piece of coal we must set fire to it, after which 

 it bums without any external assistance. We have noticed 

 that in some cases coal can also burn spontaneously, when it 

 comes into contact with the oxygen of the air. On the other 

 hand, in order to decompose carbonic acid and water, to ' un- 

 burn ' them, we must expose them to a very high temperature. 

 Formerly it was supposed that the decomposition of such stable 

 compounds was impossible without the co-operation of a third 

 body, endowed with a stiU greater affinity for oxygen, and so 

 able to sever that oxygen from hydrogen and carbon. But not 

 so very long ago the attention of chemists was drawn to pheno- 

 mena of decomposition, or dissociation, as they were called, 

 apart from the action of any third body. In order to be 

 dissociated, carbonic acid or water must be passed through red- 

 hot tubes. Under the influence of the motion thus communicated 

 to their particles, i.e. of heat, the connexion between them 

 becomes loosened, so to speak; the compounds break down 

 into their component parts, which must be immediately re- 

 moved, lest on cooling they should recombine and prevent us 

 from obtaining a complete separation. The amount of heat 

 liberated at the moment of combination and absorbed at the 

 moment of dissociation is strictly definite. There are exactly 

 as many units of heat liberated during the oxidation of a pound 

 of carbon into carbonic acid, as there are units of heat absorbed 

 during the Uberation of this pound of carbon from carbonic acid. 

 Thus we come to the conclusion that the dissociation of carbonic 

 acid, which takes place in the plant, must be accompanied by 

 the absorption of heat, of energy speaking generally, and also 

 that the amount of carbon precipitated in this way in the plant 

 may serve as a measure for tliis absorption. But whence will 

 the plant obtain this energy so essential to it ? It cannot create 

 it, because energy cannot be created. Apparently it must 

 acquire it from without. The dissociation of carbonic acid 

 in a plant can take place only on condition that there is a con- 

 tinual supply of energy from an outside source. This was the 

 condition which escaped Priestley's attention, and the discovery 



