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through space has been estimated and given in figures, by a 
profound philosopher, who, in treating of the undulations of 
light, describes it as “ an elastic medium pervading all space, 
not even excepting what is occupied by what we call 
material bodies, that is, such as have weight and which to 
us constitute the visible and tangible universe of things.” — 
(Sir John Herschel, Bart., “On Light,” in Good Words, for 
July, 1865.) 
In the works of Boerhaave the said medium is described 
as “ a calorific element, sui generis, pervading all space and 
all bodies, and being in itself of the nature of fire.” Thus 
“ a pervading element” rests on very high authority, both 
ancient and modern ; and it is of the nature of such element, 
and its agency in natural phenomena, that I propose to treat, 
and to show that the different forms of imponderable matter, 
as light, heat, electricity, and magnetism, are all essentially 
identical, and in proof of this we must look to their reciprocal 
interchanges with each other, whereby their common origin, 
in one pervading element, will be made probable, if not a 
physical certainty. 
The term formerly applied to this pervading element was 
a “ calid” or “ calorific” element, — thus conveying the idea 
of active fire or very great heat ; but they do not accord with 
the nature or properties which I ascribe to such pervading 
element, that of “ neutral or elemental heat f and the term 
latterly employed to indicate the same element is also 
inapplicable, since it only points to one of its properties, viz., 
elasticity. I have, therefore, adopted the term of “ neutral 
heat,” which is meant to convey the idea or notion that this 
element, in fact, constitutes the source, or fountain, whence 
all other forms of heat are derived, and into which they 
alternately return. This neutral element is everywhere of 
equal density, by reason of its elastic tension, and it there- 
fore cannot exert or possess the force of gravity or of inertia, 
and it must remain in a quiescent state, except in cases 
