88 Prof. Challis on a Theory of Molecular Forces. 
amongst other experimental results, I believe I may venture to 
assert that the differences of sound depend only upon the differ- 
ences in the intensities of the notes; but that the latter, under 
the above-mentioned circumstances, depend upon differences of 
hase. 
i For the present, however, I would prefer to limit the above 
assertion to the lower incidental notes, which, lying far apart in 
the scale, reach as far as the sixth and seventh. The higher 
incidental notes give discords and beats with each other; and 
when a number of such pairs of notes which give rise to beats 
are heard together, it is probably not indifferent, as far as per- 
ception is coucerned, whether the pauses of all these beats fall 
together or not. The latter, however, depend upon the differ- 
ences of the phase. Moreover, I hold it to be probable that all 
these higher dissonant incidental notes form what the ear recog- 
nizes as accompanying noises, which latter we have already ex- 
cluded from our consideration of musical tones. 
I have in another place been led to the hypothesis, that each 
nervous fibre of the auditory nerve is destined for the perception 
of notes of a particular pitch, and is excited when the note which 
strikes the ear corresponds in pitch to that of the elastic forma- 
tion* in connexion with the fibre. According to this, the percep- 
tion of different tones would reduce itself to the simultaneous 
excitation of the fibre which corresponds to the primary note, 
and of certain others corresponding to the incidental notes. 
This simple explanation could not have been given had the dif- 
ferences of phase of the lower incidental notes entered into con- 
sideration. 
oe 
XILI. A Theory of Molecular Forces. 
By Professor Cuaxuist. 
HE general Theory of Physical Forces, the principles of which 
I have indicated in previous communications, must, if it 
have a real foundation, include a theory of molecular forces ; that 
is, of the forces by which the constituent atoms of bodies are 
held in different states of aggregation—as the solid, the fluid, and 
the gaseous. The inquiry into the laws and modes of action of 
this class of forces has long engaged the attention of physicists, 
and has given rise to a great variety of special hypotheses, mostly 
of an arbitrary kind, and not referable to any general principle. 
The theory I am about to explain differs from all that have pre- 
ceded it in this respect, that it admits of no other kind of action 
than the pressure of a very elastic fluid medium (the ether), and 
* “ Des Cortischen Organs oder Borste in den Ampullen.” 
+ Communicated by the Author. 
