J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’

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                                (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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Manuscript details

Author
James Clerk Maxwell
Reference
PT/72/7
Series
PT
Date
1864
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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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