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that translatory action, which appears as attractive or repulsive 
force, may be produced by vibrations. But if this be so, it is plain 
that we have the means, without having recourse to occult qualities, 
of framing a theory of attractive and repulsive forces, inasmuch as 
these might be accounted for by dynamical effects of vibrations of 
the ether. Accordingly, since it is allowable to assume that as there 
are light-producing vibrations of the aether, there are also heat- 
producing, we can by this theory give a reason for the observed 
repulsive power of heat. 
(5.) By applying this theory of motion of translation of atoms 
caused by ethereal undulations to the case of gravity , I have found 
that this force may be attributed to the 'translatory action of 
undulations of such magnitude that they may be supposed to 
se. lar a e masses, such as the sun and the planets, without 
undergoing sensible change or retardation. The space occupied by 
the matter of the atoms must be supposed, even in the densest 
bodies, to be very small, compared to the intervening spaces. Under 
these ciicumstances it appears that both the law of the inverse 
square, and Galileo's Theorem, which were spoken of in articles 
3 and 4 as constituting the foundation of Physical Astronomy 
may be accounted for. 
(6.) A molecule, as the name implies, may be taken to be a 
mass of atoms. One molecule may be conceived to differ from 
another solely by reason of difference in the number and arrange- 
ment of. the atoms. The law of action of gravity on all bodies 
equally is most simply explained by supposing the constituent 
atoms of all bodies to be of the same size. My hvdrodynamical 
researches point to the necessity for fulfilling this condition in 
order to account for that law. 
(7.) As there are winds of the air as well as vibrations, so there 
are currents, as well as vibrations, of the ether. The ethereal 
vibrations are concerned in producing the repulsion of heat, mole- 
culai attraction, and the force of gravity. Electric Force, Galvanic 
Force, and Magnetic Force, are referable to steady ethereal currents 
m a manner I shall endeavour presently to give some idea of. 
From Hydrodynamics we learn that such currents are accompanied 
by variations of density and pressure ftom point to point of space, 
and to these variations of pressure differently produced in the three 
instances, the three kinds of force are attributable. 
(8.) ihe solid, liquid, and gaseous states of the same substance 
are lefeiable to different conditions of the action of the atomic 
lepulsion of heat and molecular attraction within a superficial 
stiatum of the substance of extremely small thickness. In the 
gaseous or aeriform state there is no superficial molecular attraction 
to control the atomic repulsion, and consequently a gaseous body, 
