528 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



It may be filled with any kind of matter, or we may endeavour to render 

 it empty of all gross matter, as in the case of Geissler's tubes and other so- 

 called vacua. 



There is always, however, enough of matter left to receive and transmit 

 the undulations of light and heat, and it is because the transmission of these 

 radiations is not greatly altered when transparent bodies of measurable density 

 are substituted for the so-called vacuum, that we are obliged to admit that the 

 undulations are those of an sethereal substance, and not of the gross matter, 

 the presence of which merely modifies in some way the motion of the aether. 



We have therefore some reason to believe, from the phenomena of light 

 and heat, that there is an {ethereal medium filling space and permeating bodies, 

 capable of being set in motion and of transmitting that motion from one part 

 to another, and of communicating that motion to gross matter so as to heat 

 it and affect it in various ways. 



(5) Now the energy communicated to the body in heating it must have 

 formerly existed in the moving medium, for the undulations had left the source 

 of heat some time before they reached the body, and during that time the 

 energy must have been half in the form of motion of the medium and half in 

 the form of elastic resilience. From these considerations Professor W. Thomson 

 has argued*, that the medium must have a density capable of comparison with 

 that of gross matter, and has even assigned an inferior limit to that density. 



(6) We may therefore receive, as a datum derived from a branch of science 

 independent of that with which we have to deal, the existence of a pervading 

 medium, of small but real density, capable of being set in motion, and of trans- 

 mitting motion from one part to another with great, but not infinite, velocity. 



Hence the parts of this medium must be so connected that the motion of 

 one part depends in some way on the motion of the rest; and at the same 

 time these connexions must be capable of a certain kind of elastic yielding, 

 since the communication of motion is not instantaneous, but occupies time. 



The medium is therefore capable of receiving and storing up two kinds of 

 energy, namely, the "actual" energy depending on the motions of its parts, and 

 "potential" energy, consisting of the work which the medium will do in recover- 

 ing from displacement in virtue of its elasticity. 



* "On the Possible Density of the Luminiferous Medium, and on the Mechanical Value of a 

 t'ubic Mile of Sunlight," Transactions of tlie Royal Society of Edinburgh (1854), p. 57. 



