(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 <s>very nearly<\s> 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 <s>take place in<\s> are those of an etherial substance, and not of the gross matter, the presence of which merely modifies in some way the motion of the ether. We have therefore some reason to believe, from the phenom[ena] of light and heat that there is an etherial 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 <s>[them?]<\s> it and affect <s>them<\s> 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 <s>partly<\s> half in the form of elastic resilience. From these considerations Professor W. Thom[son] 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. *“On the Possible Density of the Luminiferous Medium and On the Mechanical Yalue of a Cubic Mile of Sunlight Transactions of the Royal Society of Edinburgh (1854) p 57.
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J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7
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