le with such as belong to matter. As for cohesion,
it is doubtful if the term ought to be applied to such a substance. The
word itself seems to imply possible separateness, and if the ether be a
single indivisible substance, its cohesion must be infinite and is
therefore not a matter of degree. The ether has sometimes been
considered as an elastic solid, but such solidity is comparable with
nothing we call solid in matter, and the word has to be defined in a
special sense in order that its use may be tolerated at all. In addition
to this, some of the phenomena exhibited by it, such as diffraction and
double refraction, are quite incompatible with the theory that the ether
is an elastic solid. The reasons why it cannot be considered as a liquid
or gas have been considered previously.

The expression _states of matter_ cannot be applied to the ether in any
such sense as it is applied to matter, but there is one sense when
possibly it may be considered applicable. Let it be granted that an atom
is a vortex-ring of ether in the ether, then the state of being in ring
rotation would suffice to differentiate that part of the ether from the
rest, and give to it a degree of individuality not possessed by the
rest; and such an atom might be called a state of ether. In like manner,
if other forms of motion, such as transverse waves, circular and
elliptical spirals, or others, exist in the ether, then such movements
give special character to the part thus active, and it would be proper
to speak of such states of the ether, but even thus the word would not
be used in the same sense as it is used when one speaks of the states of
matter as being solid, liquid, and gaseous.

20. SOLID MATTER CAN EXPERIENCE A SHEARING STRESS, LIQUIDS AND GASES
CANNOT.

A sliding stress applied to a solid deforms it to a degree which depends
upon the stress and the degree of rigidity preserved by the body. Thus
if the hand be placed upon a closed book lying on the table, and
pressure be so applied as to move the upper side of the book but not the
lower, the book is said to be subject to a shearing stress. If the
pressing hand has a twisting motion, the book will be warped. Any solid
may be thus sheared or warped, but neither liquids nor gases can be so
affected. Molecular cohesion makes it possible in the one, and the lack
of it, impossible in the others. The solid can maintain such a
deformation indefinitely long, if the pressure does not rupture it