ETHER. 
each other, so far that the effects appear to 
us to be simple, we resolve the causes of 
motion into two ; namely, a disposition of 
bodies to come together, called attraction, 
and a disposition to recede from each other, 
called repulsion. Impulse, or the commu- 
nication of motion by apparent contact, 
will riot constitute a peculiar case, because 
we know that bodies cannot be, or are not 
in any of our observations, brought close to 
each other. But as in all our philosophising 
we endeavour to simplify the general prin- 
ciples, it becomes a question, whether the 
effects of attraction and repulsion may not 
depend upon the same cause; and as we 
have many gross instances of bodies being 
urged together by the action of fluids, it na- 
turally occurs to enquire, whether the ap- 
parent attractions in nature may not be 
caused by some fluid medium. Sir Isaac 
Newton was strongly of this opinion, as ap- 
pears by his letter to Boyle, published in 
Birch’s life of that philosopher, as well as 
by the famous paragraph at the end of his 
“ Principia,” and one of the queries at the 
end of his “ Optics,” in the preface to the 
second edition of which he remarks, that he 
does not take gravity for an essential pro- 
perty of bodies. In the query here men- 
' tioned, he proceeds upon the supposition of 
an elastic medium, pervading all space ; a 
supposition which he advances with consi- 
derable confidence, and which he supports 
by very strong arguments, deduced as well 
from the phenomena of light and heat, as 
from the analogy of the electric and magne- 
tic influences. This medium he supposes 
to be much rarer within the dense bodies 
of the sun, the planets, and the comets, 
than in the empty celestial spaces between 
them, and to grow more and more dense at 
greater distances from them, so that all 
these bodies are naturally forced towards 
each other by the excess of pressure. 
The effects of gravitation might be pro- 
duced by a medium thus constituted, if its 
particles were repelled by all material sub- 
stances, with a force decreasing like other 
repulsive forces, simply as the distances in- 
crease ; its density would then be every 
way such as to produce the appearance of 
an attraction varying like that of gravita- 
tion : such an ethereal medium would there- 
fore have the advantage of simplicity, in the 
original law of its action, since the repul- 
sive force which is known to belong to all 
matter^ would be sufficient, when thus mo- 
dified, to account for the principal pheno- 
mena of attraction. 
It may be questioned, whether a medium 
capable of producing the effects of gravita- 
tion in this manner, would also be equally 
susceptible of those modifications which we 
have supposed to be necessary for the trans- 
rhission rif light : in either case it must be 
supposed to pass through the apparent sub- 
stance of all material bodies, with the most 
perfect freedom, and there would, there- 
fore, be no occasion to apprehend any diffi- 
culty from a retardation of the celestial mo- 
tions ; the ultimate impenetrable particles 
of matter, being perhaps scattered as thinly 
through its external form, as the stars are 
scattered in a nebula, which has still the 
distant appearance of an uniform light, and 
of a continuous surface i and there seems 
no reason to doubt the possibility of the 
propagation of an undulation through the 
Newtonian medium; with the actual velo- 
city of light. It must be remembered that 
the difference of its pressure is not to be es- 
timated from tlie actual bulk of the earth, 
or any Other planet alone, but from the ef- 
fect of the sphere of repulsion of which that 
planet is the centre ; and w’e may then de- 
duce the force of gravitation from a medium 
of no very enormous elasticity. 
A similar combination of a simple pres- 
sure with a variable repulsion, is also ob- 
servable in the force of cohesion ; and Dr. 
Young, in his Lectures, remarks, that sup- 
posing two particles of matter floating in 
such an elastic medium, capable of produc- 
ing gravitation, to approach each other, 
their mutual attraction would at once be 
changed from gravitation to cohesion, upon 
the exclusion of the portion of the medium 
intervening between them : this supposition 
is, however, as he adds, directly opposite to 
that which assigns to the elastic medium 
the power of passing freely through all the 
interstices of the ultimate atoms cohering 
in this manner ; but that, as we see some ef- 
fects so nearly resembling them, which are 
Unquestionably produced by the pressure 
of the atmosphere, we can scarcely avoid 
suspecting that there must be some ana- 
logy in the causes. 
Two plates of metal, which cohere enough 
to support each other in the open air, will 
often separate in a vacuum. When a boy 
draws along a stone by a piece of wet lea- 
ther, the pressure of the atmosphere seems 
to be materially concerned. The well- 
known experiment, of the two exhausted 
hemispheres of Magdeburg, affords a still 
more striking instance of apparent cohesion 
derived from atmospherical pressure ; and 
