elasticity. 
<;tass and order. Essential character; co- 
rolla five-petalled ; drupe ovate, with a 
two-celled nut There are two species, of 
which E, 01 ientale is a moderate-sized twig- 
gy shrub, or tree, a native of the oriental 
regions : leaves ovate lanceolate, smooth, 
slightly waved, sometimes inclining to a 
subserrated appearance on the upper parts 
of the shoots ; flowers borne towards tlie 
ends of the branches, of a pale green colour, 
supported on shortish pedicles, each of which 
springs from a longer common pedicle. 
ELASTICITY, that disposition in bodies 
by which they endeavour to restore them- 
selves to the posture from whence they 
were displaced by any external force. The 
principal phenomena observable in elastic 
bodies are : 1. Tliat an elastic body (i. e. a 
body perfectly elastic, if any such there be) 
endeavours to restore itself with the same 
force with which it is pressed or bent. 2. An 
elastic body exerts its force equally towards 
all sides ; though the effect is chiefly found 
on that side where the resistance is weakest, 
as is evident in the case of a gun exploding 
a ball, a bow-shooting out an arrow, &c. 
3. Elastic bodies, in what manner soever 
struck, or impelled, are inflected, and re- 
bound after the same manner: thus a bell 
yields the same musical sound, in what 
manner or on what side soever it be struck ; 
the same of a tense or musical chord ; and a 
body rebounds from a plane in the same 
angle in which it meets or strikes it, making 
the angle of incidence equal to the angle of 
reflection, whether the intensity of the 
stroke be greater or less. 4. A body per- 
fectly fluid, if any such there be, cannot be 
elastic, if it be allowed that its parts cannot 
be compressed. 5. A body perfectly solid, 
if any such there be, cannot be elastic ; be- 
cause, having no pores, it is incapable of 
being compressed. 6. The elastic proper- 
ties of bodies seem to differ, according to 
their greater or less density or compactness, 
though not in an equal degree : thus, metals 
are rendered more compact and elastic by 
being hammered ; tempered steel is much 
more elastic than soft steel ; and the density 
of the former is to that of the latter as 7809 
to 7738 : cold condenses solid bodies, and 
renders them more elastic ; whilst heat, that 
relaxes them, has the opposite effect : but, on 
the contrary, air, and other elastic fluids, are 
expanded by heat, and rendered more clastic. 
Sqme philosophers account for elasticity 
from the principles of corpuscular attraction 
and repulsion : thus, if a steel spring, wire, 
or piece of very tlrin glass, be bent out of its 
natural position, tlie particles on the conve.x 
part are forced from the intimate union they 
had before ; and, on the concave part, they 
are forced nearer together, or harder upon 
each other, than in the natural state: in 
both which cases there will be a considerable 
resistance to overcome, and consequently 
require a superior force. During this state 
of the particles, they may be said to be un- 
der a sort of tension on one side, and com- 
pression on the other; and since by this 
force they are not drawn out of each others 
attraction, as soon as the force is remitted 
or ceases to act, the attractive power re- 
duces the particles, and unbends the wire. 
Kow it is well known, that many substances 
are composed of such fibrous parts or fila- 
ments which resemble fine wires, and are 
interwoven and disposed in such a manner, 
as in sponge, for instance, that they cannot 
be compressed without being bent or wrest- 
ed from their natural position ; whence all 
bodies will in such cases exert a spring or 
force to restore themselves in the same man- 
ner that the bent wire did. Others attri- 
bute the elasticity of all hard bodies to the 
force of the air included within them ; and 
so they make the elastic force of the air the 
principle of elasticity in all other bodies. 
See Pneumatics. 
All substances that we know of are in some 
degree or other elastic, but none of them 
perfectly so; such are most metals, semi- 
metals, stones, and animal and vegetable 
substances, however they may differ in de- 
gree. 
We may consider all elastic bodies to be 
made up of such strings or fibres as A B 
(Plate IV. Miscel. fig. 9.) of rather of elastic 
strata parallel to each other, represented by 
A B in the ball D C. If this ball be struck 
at D by a hard or elastic body, all tlie stra- 
ta will be bent in towards C, as expressed 
by the dotted lines, whilst the ball is flat- 
tened or dented at D. But the strata 
quickly restoring themselves, the surface of 
the ball re-assumes its first figure, and that 
more or less exactly, according as the elas- 
ticity is more or less perfect. 
The great law of perfectly elastic bodies, 
is, that their relative velocity will remain 
the same before and after collision ; that is, 
perfectly elastic bodies will recede from 
one another after the stroke with the same 
velocity that they came together. Many 
curious phsenomena may be explained from 
this property in bodies. , 
If the ivory ball A, (fig. 10.) weighing two 
ouncesj strike with the velocity 16 against 
