142 THE LIFE OF THE PLANT 



elements are only yielding to a mutual attraction, just 

 as these balls are, but having knocked against each 

 other they are warmed, and give off heat. 



The phenomena of decomposition are quite differ- 

 ent. In order to decompose a chemical compound we 

 must expend a certain amount of energy. In our 

 illustration the same quantity of energy is required to 

 drive the balls away from each other as will be mani- 

 fested at the impact of the bodies against each other, 

 when I withdraw my hands. This equality in the 

 quantity of energy employed in decomposition and 

 liberated during combination is easily illustrated by 

 our mechanical contrivance. In order to move one 

 ball away from the other I must raise it, must over- 

 come the force of gravity. I estimate the quantity 

 of energy expended in this instance by my work, and 

 this work is measured by the product of the weight of 

 the ball multiplied by the height to which it is raised. 

 But just at the moment when the balls hit each other, 

 the falling ball is endowed with energy sufficient to 

 raise to the same height a ball of the same weight. 

 We draw this conclusion from the fact that if it had 

 not come into contact with the other ball it would 

 have swung like a pendulum to the other side, and 

 raised itself to the same height, i.e. it would have raised 

 its own weight to the same height from which it has 

 just fallen. In the same way, in order to break up a 

 compound, to overcome the affinity of two chemical 

 bodies and separate them, it is necessary to employ 

 the same amount of energy as was liberated at the 

 moment of their combination. If a certain amount 

 of carbon burning in oxygen gives off, say, one thousand 

 units of heat, then in order to set free this carbon from 

 the carbonic acid which is formed, and overcome its 

 affinity for the oxygen, we must of necessity employ 

 the same one thousand units of heat. We can, indeed, 

 decompose carbonic acid, i.e. liberate its carbon, as we 



