i88i.J 
The Source of Electric Energy. 
381 
mission of radiance through the substance concerned, the 
other a resistance to a return of the ray through the air. If 
the former resistance be the least the ray will be transmitted, 
and we call the substance a transparent. If the latter be 
the least the ray will be reflected, and we call the substance 
an opaque. 
But this power of resistance has its peculiarities. Some 
substances resist all radiant vibrations, and are completely 
opaque. Others resist one range and are permeable to 
another. Some, for instance, permit the light rays to pass, 
and are opaque to the lower heat rays. Others, as rock salt, 
are highly transparent to these lower heat rays. Some 
again, among the opaques, are permeable to a slight degree, 
as gold for instance. Thus, whether a radiant beam, or any 
portion of it, shall be repelled through the air, or shall pass 
through the body upon whose surface it falls, depends upon 
the relative degree of resistance to transmission or refledtion. 
Perhaps where resistance to both transmission and reflection 
is great the resistance to absorption may be more fully 
overcome, and vibrations only slightly in accord with the 
heat vibrations be received as heat. It seems, indeed, as if 
it might be but a question of relative resistances, these 
resistances being of three or more separate kinds, and the 
motive energy being divided up among them according to 
the special vigour of each resistance to each component of 
the ray. 
These considerations in regard to light are not without 
their bearing upon the question of eledtric energy. For 
although many substances are completely opaque to the 
whole range of etherial vibrations, it does not follow that 
they must be opaque to all possible vibrations. The vibra- 
tions which constitute light and radiant heat occupy but a 
limited portion of the whole range of possible vibratory 
pitch. Between the lowest heat ray and the highest vibra- 
tion of sound there is a vast void, capable of containing 
vibrations immensely varying in pitch. The whole range 
between several thousands and several millions of millions 
of vibrations per second is apparently unoccupied, and into 
this wide space it would not be safe to conclude that no 
phenomena of motion can enter. It may, indeed, be 
possible that every substance is transparent to vibratory 
waves of a certain limited range of pitch, and opaque to all 
above and below this range. The range of vibrations to 
which ether is transparent is really limited, so far as indica- 
tions go. Of course, it is possible that the light and heat 
