92 The Rev. J. Challis on Capillary Attraction 
The way in which the above hypothesis may be conceived 
to account for the fluidity of fluids may be stated as follows: 
The distinguishing characteristic of fluids is the facility with 
which the relative positions of different parts may be made to 
change, excepting when the forces applied tend to compress 
the whole mass into a smaller space. Now, since it is a con- 
sequence of our hypothesis, that the repulsive force of each 
atom varies very rapidly with the distance, and the attractive 
force slowly, the above-mentioned property will be accounted 
for by saying, that any cause which compresses in a very slight 
degree any portion of the atoms, by diminishing their mutual 
distances, calls into play their repulsive action, without sen- 
sibly affecting their attractions on each other, or on particles 
more remote, so that they will be ready to obey any inequality 
of repulsion without drawing surrounding particles with them. 
Suppose, for instance, a plate of small but finite thickness 
were dipped in a fluid with its planes vertical. When it just 
begins to enter, it compresses the particles immediately under, 
and by diminishing their mutual distances calls forth a repul- 
sion, which again compresses and puts in motion the conti- 
guous particles: these in like manner act on the next in suc- 
cession, and so on till the whole mass is made to yield to the 
immersion. If at the same time a sensible attractive force 
were excited by the compression, the surrounding particles 
would be drawn towards the compressed parts, and the effect 
on those at the surface would be a depression near the im- 
merging plate, such as we know to take place when a solid is 
dipped in a semz-fluid substance. An obstacle would thus be 
presented to the immersion of the plate, the same in kind as that 
presented by a solid, the rigidity of which, according to this 
theory, will be owing to a great degree of energy in the mo- 
lecular attractions. But when the attractive forces are very 
feeble, there will be little resistance to the immersion besides 
that arising from the inertia of the particles, and the fluidity 
will consequently be nearly perfect. This distinctive pro- 
perty which fluids possess of being readily divided, I have 
elsewhere observed, may be conveniently employed as a prin- 
ciple from which the fundamental equations relating to their 
equilibrium or motion may be derived. 
Some mathematical consequences, which, as I am about to 
show, follow from the views expounded above, will also serve 
to confirm the initial hypothesis. Admitting that the varia- 
tion of density at the surfaces of fluids has small influence on 
any effects resulting from their molecular attractions, and that 
the molecular repulsions are taken account of by supposing 
