378 The Source of Electric Energy. [July, 
In regard to heat and to radiant energy, science gives 
definite answers to these questions. Heat it declares to be 
a persistent motion of molecules, either independent of 
special control, as in the gas, or controlled by attraction, as 
in the solid and liquid. Radiance it declares to be a rapidly 
transmitted vibration of molecules or of etherial particles, 
through whose agency motive energy is direCtly transferred 
across wide intervals of space. This explanation, however, 
is questioned ; and the latest theory, that of Clerk Maxwell, 
declares that there is no direCt transfer of energy, but that 
it is, in some manner, indirectly transferred. In regard to 
electricity, the question yet remains unanswered. Is it a 
direCtly or an indirectly transmitted energy ? Is it a rapidly 
transferred vibration, as in the first explanation of radiant 
energy ? Or is it a result of some distinct and peculiar mode 
of motion ? These are questions which still lack a solution. 
As to the conditions under which radiant energy is trans- 
formed into heat, we are not without a satisfactory concep- 
tion. When a beam of light or radiant heat, consisting of 
vibrations of greatly varied rapidity, falls upon the surface 
of a body, a certain portion of this energy is retained by the 
body, and the remainder either transmitted or reflected. 
The retained energy becomes heat, and little doubt is enter- 
tained as to the principle of its conversion into heat. The 
particles of the body aCted on are at a certain tension, 
dependent upon their weight, their attractive connections, 
and the pressure to which they are subjected. Their heat 
vibrations, therefore, take place at a certain fixed pitch, on 
the same principle that a stretched string vibrates at a pitch 
in strict accordance with its tension. When the rays of 
light impinge upon these molecules every vibration which 
is in harmony with the pitch of molecular vibration is 
retained, and becomes heat. Every vibration in discord 
with that of the molecules is rejected. The principle is 
precisely the same as that of a string stretched to sound a 
certain note, near which a second string is made to vibrate. 
If the note of the second string accords with that of the first, 
this latter will take up the vibration. If it be discordant, 
the first string will remain unaffected. 
If, now, we come to consider the conditions under which 
heat becomes electricity, we find indications of a similarity 
to those just considered. Every substance has its normal 
pitch of molecular vibration, which differs, however, with 
every difference in the tension of the molecules. If two 
homogeneous surfaces be brought into contact heat is regu- 
larly conducted from one to the other. And if a substance 
