TYPES OF ENERGY. 307 



816. Light and assimilation proper. Energy has been defined 

 as the power of doing work. Of this there are two types : the 

 energy of actual motion (sometimes termed kinetic), and the 

 energy of position (known as potential). The illustration of 

 their difference is usually given as follows : A ball thrown up- 

 wards has the power of overcoming the force of gravity tending 

 to pull it down, and possesses energy of motion ; suppose the 

 ball at the end of its course is lodged upon some projecting 

 shelf, then its energy of motion disappears, and it now pos- 

 sesses energy of position. Whenever it is dislodged, it will fall 

 with the same power which was required for its ascent. From 

 this and similar examples it is plain that one form of energy 

 can be changed into another; when one seems to disappear, 

 it has in fact merely been converted into some other. 



817. These t} r pes of energy are to be found in molecules as 

 well as in masses of matter. It is held that all molecules of all 

 matter are in a state of motion, invisible, but none the less real. 

 One form of such invisible kinetic energy is heat, and another 

 is radiant light, where the energy of motion is embodied in the 

 vibrations or undulations of the ethereal medium. A third form 

 is that of electrical separation ; and still another, with which 

 Physiology deals especially, is known as chemical separation, of 

 which a familiar illustration may be given : An atom of oxygen 

 has so strong an attraction for one of carbon, that if the two 

 are united, it is difficult to separate them, the force required 

 to do this being comparable to that demanded to raise a weight 

 to a certain height. As in the latter case the weight held in 

 its raised position represents by that position the force which 

 was employed to raise it, so the separated atoms represent 

 energy of position ready to be again converted into energy of 

 motion. 1 



obvious ; and why it is that the waves of ether beating with greater or less rapid- 

 ity on the retina should produce such sensations as those of violet, blue, yellow, 

 or red, the physiologist is wholly unable to explain. We have, however, an 

 analogous phenomenon in sound, for musical notes are simply the effects of 

 waves of air beating in a similar way on the auditory nerves ; and, as is well 

 known, the greater the frequency of the beats, or, in other words, the more 

 rapid the oscillations of the aerial molecules, the higher is the pitch of the note. 

 Red color corresponds to low, and violet to high notes of music, and the gra- 

 dations of color between these extremes, passing through various shades of 

 yellow, green, blue, and indigo, correspond to the well-known gradations of 

 musical pitch" (Cooke: Chemical Philosophy, 1882, p. 189). 



1 It is seldom that one of these forms of molecular energy when exhibited in 

 the phenomena of living beings is not associated with some other form. Thus 



