WILLIAM G. STEVENSON. 19 
eludes the senses, is, nevertheless, proved to be a mate- 
rial reality—although its nature has not been defined. 
The phenomena of light, as interpreted by the undu- 
latory theory of Young, establish the fact that ‘‘light is 
not a substance, but a process going on in a substance ;’’ 
and, reasoning from verified data, proof is given that 
this substance which is capable of transmitting energy, 
must itself possess the physical properties of elasticity 
and density, and, therefore, be a form of matter. 
But what form? Here analogical reasoning leads to 
uncertainty ; for a medium that fills all space and trans- 
mits energy, even in an atmospheric vacuum, suggests 
the idea of ‘‘extreme tenuity,’’ which indicates a ‘‘ den- 
sity that is almost infinitesimal ;”’ this means that the 
particles, which constitute this medium, are, themselves, 
not only ‘‘mathematical zeros in weight,’’ but, also, that 
they are separated from each other by regions of empty 
space. A medium having such a molecular constitution, 
however, cannot transmit the energy of light, and this 
contradiction of facts necessitates a rejection of the idea 
of a medium of-extreme tenuity. On the other hand, 
the fact that light is transmitted at the rate of one hun- 
dred ninety thousand miles in a second, compels us 
to regard this transmitting medium which—to the senses 
—is so ‘‘impalpable, invisible, and imponderable,’’ as in 
reality ‘‘a medium infinitely more compact than the 
most solid substances which can be felt and weighed.”’ 
It forced Sir John Herschel to conceive it not as an air, 
nor as a fluid, but as a solid—‘‘in this sense, at least, 
that its particles cannot be supposed as capable of inter- 
changing places, or of bodily transfer to any measurable 
distance from their own special and assigned locality in 
the universe ;’’ this would give to this medium a consti- 
tution not molecular but continuous, and make it a 
solid whose density is greater than steel. 
The earth is thus united to the worlds in space by 
