ELEMENTARY VITAL PHENOMENA 209 



III. THE PHENOMENA OF TRANSFORMATION OF ENERGY 



A. THE FORMS OF ENERGY 



For a long time natural science has distinguished different 

 forces which bring about the phenomena of motion in nature. In 

 the scientific sense force is nothing but an expression for the 

 cause of motion, for we know nothing concerning it except that it 

 causes motion. Sense-perception is not force, it is merely motion. 

 Accordingly, since early times, wherever different kinds of motion 

 have been seen, different kinds of force have been assumed. It 

 thus came about in time that a large number of forces were 

 distinguished, which could not in any way be compared with one 

 another, because some kinds were only special cases of others, some 

 were combinations of several kinds, and some were not forces at 

 all. The force of gravity, muscular force, and the force of will were 

 all spoken of. This condition of things has not yet wholly dis- 

 appeared. The forces that physics still recognises are not equivalent 

 things, and little light has been thrown, even yet, upon the rela- 

 tions of certain ones to others. 



In recent times, in accordance with the usage of Th. Young 

 and Thomson, the old and easily misunderstood name " force " has 

 been replaced by the term " energy," and what earlier were termed 

 different forces are termed now different forms of energy. Thus, 

 physics now recognises in general the following forms of energy : 



1. Chemical energy (chemical affinity, attraction of atoms). 



2. Molecular energy (cohesion, adhesion, attraction of molecules). 



3. Mechanical energy (pressure, traction, thrust). 



4. Energy of gravitation (gravity, attraction of masses). 



5. Thermal energy (heat). 



6. Photic energy (light). 



7. Electrical energy (electricity, galvanism). 



8. Magnetic energy (magnetism). 



We will glance at these individual forms. 



Modern natural science, as is well known, conceives the physical 

 world to be composed of extremely small particles ; it terms the 

 particles that cannot be divided further without losing their proper- 

 ties, molecules, and those that compose the molecule and are indivis- 

 ible, atoms. Chemical energy is that form of energy by which atoms 

 attract one another in order to form a molecule ; molecular energy 

 that form by which molecules attract one another in order to form 

 masses. If a mass is in motion and strikes against another mov- 

 able body, it puts this likewise into motion if the impact be strong 

 enough. The form of energy that puts in motion the body that is 

 struck is mechanical energy. Further, masses attract one another, 



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