ENERGY IN PROTOPLASM. 33 



set free. Physicists distinguish two forms of energy, viz., 

 kinetic energy, the energy of motion, and potential or latent 

 energy, the energy of position. The former is active, the latter 

 passive. What is called the liberation of energy consists simply 

 in the transformation of potential into kinetic energy. Thus, a 

 stone lifted above the ground possesses potential energy because 

 its position enables it to fall, and in falling it can do work or 

 overcome resistance. When the stone is let fall, its former 

 energy of position becomes energy of motion ; and in like 

 manner every form of kinetic energy is produced by the liber- 

 ation of potential energy. 



Now the energy which protoplasm expends in doing work is 

 potential energy derived from its food. Protoplasm takes in food- 

 matters rich in energy and transforms them into its own substance, 

 thus acquiring a store of intrinsic potential energy. This energy 

 is ultimately set free by complex chemical changes taking place 

 in the living protoplasm itself. Although these changes are not 

 yet fully understood, they are known to consist, broadly speaking, 

 in the breaking-down or decomposition of complex unstable 

 compounds into simpler and stabler ones under the influence of 

 oxygen ; and in the long run they are complicated processes 

 of combustion or oxidation (cf. p. 4). It is a well-known 

 fact that energy is set free whenever strong chemical affinities 

 are satisfied at the expense of weaker ones; that is, whenever 

 the elements unite to form compounds, or whenever an unstable 

 compound is resolved into one or more stabler ones.* In lifeless 

 matter such processes may or may not be processes of oxidation ; 

 in living matter they are probably in the long run always pro- 

 cesses of oxidation. 



This will be rendered clearer by a few illustrations. The 



*/ 



energy (or power) which drives a steam-engine is first set 

 free in the furnace by the union of the fuel with the oxygen 

 of the air that is, by a process of oxidation in which the mutual 

 affinities of oxygen and the carbon of the fuel are satisfied. The 

 energy which propels a cannon-ball is potential in the gunpowder 

 and becomes active at the moment of explosion. Gunpowder con- 



* Conversely, kinetic energy is used up or made potential whenever stable 

 compounds are converted into those less stable, as happens, for example, in the 

 decomposition of carbon dioxide by sunlight in green plants. 

 3 



