THE FORMS OF ENERGY. 113 



giving rise to sound. If a wire is stretched between two points and 

 pulled transversely, we have strain energy ; but, on letting the wire go, 

 it gives a musical note. Or, if the prongs of a tuning fork are pinched 

 together, and then freed, the fork sounds its note. The strained body 

 is observed to oscillate, and an interchange of strain and kinetic energies 

 takes place in it. The oscillations gradually die away, and their energy 

 is transferred to the surrounding air or other bodies, where it produces 

 the peculiar condition which is capable of affecting our sense of hearing. 

 We know that this condition consists of a combination of motion and 

 strain travelling out in wave form from the strained source through the 

 surrounding medium. There is, therefore, in the waves a mixture of 

 kinetic and strain energies, forms already recognised. But since a 

 special sense that of hearing is affected by the combination, we give 

 it a special name, that of Sound Enenjy. 



Light Energy, a Particular Case of Radiant Energy. When 



a body has been raised to a sufficiently high temperature, it becomes 

 incandescent, i.e. it is sending out light through the surrounding 

 medium. Meanwhile there is a loss of heat in the body, for if the 

 supply of heat is cut off, the body is observed to cool. Conversely, if 

 light is allowed to fall on an opaque surface, the surface is found to 

 be heated. We are familiar with this heating effect in the warmth 

 derived from sunlight, or from the light of a glowing fire. We know 

 that light is some modification of the medium through which it passes, 

 propagated out in waves from the source with a definite velocity, and 

 this modification on reaching our eyes affects our sense of sight. We 

 also know that the waves must be of lengths within certain narrow 

 limits, in order that they may affect us as light, somewhere between 

 300 and 800 millionths of a millimetre. But there are other waves 

 sent out from bodies, similar in kind to light waves, of lengths both above 

 and below these limits to which the retina is insensitive. We include the 

 whole series of waves under the general term of " radiation." Since their 

 propagation from a source accompanies loss of heat, and their absorption 

 by a body accompanies gain of heat, we must regard them as possessing 

 a form of energy which we term Radiant Energy. Formerly this energy 

 was described as radiant heat, but it is much more convenient to reserve 

 the term heat for the heat energy in the radiating body, or in the 

 heated receiving body, and to use "radiant energy" for the form into 

 which the heat energy changes when associated with the wave motion 

 which travels out into the surrounding medium. 



Electrical Energy. Many bodies on being rubbed are observed 

 to possess new mechanical properties. If a piece of sealing-wax is 

 rubbed with fur, or if a piece of very dry paper is stroked with the 

 finger-nails, it is found that small bodies, such as small pieces of paper, 

 are urged towards the rubbed surface. In other words, we have a 

 development of kinetic energy not accounted for by the immediate 

 disappearance of potential, strain, or heat energy. We suppose, there- 

 fore, that we have here a new form of energy, which we term Electrical, 

 and by making this supposition we may connect the kinetic energy of 

 the moving small bodies with the kinetic energy of the arm when 

 rubbing the electrified body, the electrical energy being the intermediate 

 form. It is slightly more difficult to rub the body which is being 



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