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PROFESSOR CLERK MAXWELL OX THE ELECTROMAGNETIC FIELD. 
(2) The mechanical difficulties, however, which are involved in the assumption of 
particles acting at a distance with forces which depend on their velocities are such as 
to prevent me from considering this theory as an ultimate one, though it may have been, 
and may yet be useful in leading to 1 the coordination of phenomena. 
I have therefore preferred to seek an explanation of the fact in another direction, by 
supposing them to be produced by actions which go on in the surrounding medium as 
well as in the excited bodies, and endeavouring to explain the action between distant 
bodies without assuming the existence of forces capable of acting directly at sensible 
distances. 
(3) The theory I propose may therefore be called a theory of the Electromagnetic Field , 
because it has to do with the space in the neighbourhood of the electric or magnetic bodies, 
and it may be called a Dynamical Theory, because it assumes that in that space there is 
matter in motion, by which the observed electromagnetic phenomena are produced. 
(4) The electromagnetic field is that part of space which contains and surrounds 
bodies in electric or magnetic conditions. 
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 sether. 
We have therefore some reason to believe, from the phenomena of light and heat, 
that there is an sethereal medium filling space and permeating bodies, capable of being 
set in motion and of transmitting that motion from one part to another, and of com- 
municating 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 infe- 
rior limit to that density. 
(6) We may therefore receive, as a datum derived from a branch of science inde- 
pendent 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 transmitting 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 
* “ On the Possible Density of the Luminiferous Medium, and on the Mechanical Value of a Cubic Mile of 
Sunlight,” Transactions of the Royal Society of Edinburgh (1854), p. 57. 
