1868. | The Modern Aspects of Physical Science. 335 
round the armatures of electro-magnets, it is converted, according to 
the prevailing hypothesis, into magnetic, and this again into electric 
energy. “This being transmitted between carbon electrodes, an 
immense amount of light and heat is produced by molecular friction 
at the point of great resistance to the passage of the current; and 
these are produced at the expense of electric energy, as proved by 
the loss of current. Here, then, we have the final transmutation 
of electric into thermic and photic energy, the latter beg so intense 
as to have thrown a shadow across the brightest sunbeam, and to 
produce an amount of illumination unattainable by any other known 
means.” —Brooke. 
By the “undulatory theory” we believe we explain not only 
the phenomena of Light but those of Heat and Electricity. With 
some misgivings we venture to ask, Have we not been advanced to 
this by a system of reasoning from analogy, and are analogies 
always to be trusted? Sound, we know, is developed by the vibra- 
tion of material atoms ; where there is matter in a state of extreme 
attenuation, it is scarcely possible to convey through it any audible 
sound, and silence reigns supreme in even the imperfect vacuum which 
we can produce. Since, then, sound is proved to require a medium 
through which its waves may be propagated, what kind of medium can 
we imagine to be all diffusive and all penetrating, in which the waves 
of Light, Heat, and Electricity can be generated, and through which 
they can be propagated across the immensities of space and amidst 
the interstices of a material body? The wave motions of Light 
and Heat are supposed to take place in an infinitely elastic, imper- 
ceptible, imponderable medium, which pervades space and fills the 
interstices between the atoms of matter. This is the “ther” of 
modern philosophers. Some evidence of the existence of an atten- 
uated medium of some kind, in the stellar regions, is obtained by the 
observations of the periodic retardation of Encke’s comet. Although 
this has been the received idea since the general adoption of the 
wave theory, it is now giving place to the hypothesis of the vibra- 
tory disturbances of the material atoms themselves; the kind of 
vibration—motion—determining the kind of physical energy which 
becomes the object of sensuous perception. This hypothesis is 
very strongly put before us in “ Heat considered as a Mode of 
Motion;” and in the numerous essays which have within the last 
few years been given to the world by the philosophers of the meta- 
physical school, especially such as are tinctured with the German 
modes of thought. It is not our purpose to accept or reject either 
hypothesis—they have been, and they may continue to be, useful 
as hypotheses; but we would not have them advanced to the dig- 
nity of theories until all that now appears contradictory or inade- 
quate in them is removed, or by extension placed in a more satis- 
factory position. That all the physical energies become sensible 
