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production is very rare if things continue as 
they are. But what is exceedingly surpris- 
ing is, that they do not; for, upon visitinc 
tire lobster maimed in these barren and uir 
happy articulations, at the end of two or three 
days, all the other articulations are found 
broken off to the fourth; and it is suspected 
they have performed the operation on them- 
selves, to make the reproduction of a leg 
certain. 
The part reproduced is not only perfectly 
like that retrenched, but also, in a certain 
space of time, grows equal to it. Hence it is 
that we frequently see lobsters which have 
their two large legs unequal, and that in all 
proportions. This shows the smaller leg to 
- be a new one. , 
A part thus reproduced being broken, 
there is a second reproduction. The sum- 
: mer, which is the only season of the year 
when the lobsters eat, is the most favourable 
time for the reproduction. It is then per- 
formed in four or iive weeks ; whereas it takes 
up eight or nine months in any other season. 
The small legs are sometimes reproduced, 
but more rarely, as well as more slowly, than 
the great ones ; the horns do the same. The 
experiment is most easily tried on the com- 
mon crab. 
REPTILES, in natural history, an order 
of amphibia, the character of which is, that 
they breathe through the mouth ; have feet, 
and flat naked ears, without auricles. There 
are five genera of reptiles, viz. 
Testudo 
Draco 
Lacerta 
Rana 
Siren. 
REPULSION, in physics, that property 
in bodies, by which, if they are placed just 
beyond the sphere of each other’s attraction 
of cohesion, thev mutually fly from each 
other. 
That there is a force which opposes the 
approach of bodies towards each other, and 
! which tends to separate them farther from 
each other, is obvious from the slightest view 
of the phenomena of nature. When we pre- 
sent the north pole of a magnet A to the same 
pole of another magnet B, suspended on a 
pivot, and at liberty to move, the magnet B 
recedes as the other approaches ; and by fol- 
lowing it vvitli A at a proper distance, it may 
be made to turn round on its pivot with con- 
siderable velocity. In this case there is evi- 
dently some force which opposes the ap- 
proach of the north poles of A and B, and 
which causes the moveable magnet to retire 
i before the other. There is then a repulsion 
between the two magnets, a repulsion which 
increases with the power of the magnets ; and 
this power has been made so great, by a pro- 
per combination of magnets, that all the force 
! of? a strong man is insufficient to make the two 
; irprth poles touch each other. The same 
repulsion is equally obvious on electrical bo- 
dies ; and indeed it is by means of it alone 
that the quantity of electricity is measured 
by philosophers. If two cork balls are sus- 
pended from a body with silk threads, so as to 
| ^touch each other ; if we charge the body with 
!j ’electricity, the cork balls separate froni each 
I other, and stand at a distance proportional 
to the quantity of electricity with which the 
' body is charged; the balls of course repel 
; each other. See Electricity. 
REP 
But it is not in electric and magnetic bo- 
dies only that repulsion is perceived. New- 
ton has shewn that it exists also between two 
pieces of glass. He found that when a con- 
vex lens is put upon a flat glass, it remains 
at the distance of the T | T th part of an inch, 
and a very considerable pressure is required 
to diminish this distance; nor does any force 
which can be applied bring them into actual 
mathematical contact ; a force may indeed 
be applied sufficient to break the glasses in 
pieces; but it may be demonstrated that it 
does not diminish their distance much beyond 
^ ie Tooo^ 1 P art °f an inch. There is, there- 
fore, a force of repulsion which prevents the 
two glasses from touching each other. 
That the particles of air repel each other 
is evident; for a considerable force is requir- 
ed to keep them as near each other as we 
find them at the surface of the earth ; and 
when this force is removed, they separate 
from each other, that is to say, the air ex- 
pands. Nor is it known how far this expan- 
sion extends. Air has been artificially ex- 
panded to 3000 times its usual built, and 
doubtless at great heights in the atmosphere 
its expansion is still much greater than that. 
On the other hand, air may be forcibly con- 
densed, that is to say, its particles may be 
made to approach nearer each other ; but a 
considerable force is required to produce this 
effect; and this force increases nearly as the 
density: it it is removed, the particles again 
separate, and the air assumes its former bulk. 
What are the limits of this condensation is 
not known; but air has been forcibly com- 
pressed to 1000th part of its natural bulk. 
I hus we see that the particles of air may be 
made to approach 16 times nearer each other. 
The elasticity of air, or the effort which it 
makes when compressed to resume its former 
bulk, is evidently the consequence of a re- 
pulsive force which its particles exert. All 
gaseous fluids possess the same repulsive 
force, and are indebted to it for their elas- 
ticity. 
The particles of stilid elastic bodies likewise 
repel each other ; for they also, when forcibly 
compressed, resume their former size, and of 
course their particles repel each other. It 
has been demonstrated by philosophers, that 
all liquids are capable of a certain degree of 
compression, and that when the compressing 
force is removed they resume their former 
bulk; consequently the particles of these bo- 
dies also repel each other. 
All bodies then possess a repulsive force, 
which exerts itself either at sensible distances 
or at insensible distances; of course the repul- 
sions may be divided into two classes. 
The only sensible repulsions with which 
we are acquainted, take place at small dis- 
tances. They may be reduced to two kinds, 
namely, electricity and magnetism. It has 
been ascertained, that bodies possessed of 
the same kind of electricity repel each other; 
and likewise the same magnetic poles of bo- 
dies repel each other; while, on the other 
hand, differently electrified bodies, and the 
different poles of magnetic bodies, attract each 
other. Repulsion increases, as far as has 
been ascertained, inversely as the square of 
the distance ; consequently, at the point of 
contact it is infinite. 
Insensible repulsion is most conspicuous in 
elastic fluids, as air and the eases; but it is 
4 B 2 
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exhibited also by elastic bodies in general. 
In these, if a judgment can be formed from 
the experiments on air, the repulsion in- 
creases nearly at the rate of — 
Insensible repulsion may either be a force 
inherent in the particles of bodies, or it may 
belong exclusively to some particular body 
combined with these particles. The first of 
these hypotheses seems to have been adopt- 
ed by Newton. 
Other philosophers have supposed that re- 
pulsion is not a property inherent in all mat- 
ter, but confined to a peculiar substance 
which has been generally considered as ca- 
loric. According to this hypothesis, there 
are two kinds of matter, one whose particles 
attract, another whose particles repel. Let 
us call the first cohesive matter, and the other 
caloric ; and let us suppose also, what must be 
the case, that cohesive matter and caloric at- 
tract each other with a certain force, in cer- 
tain circumstances. This will explain the 
expansive power ol caloric, which combining 
with the particles of other cohesive matter, 
destroys the cohesion of those particles, and 
acts upon the body as a repulsive force ; and 
this appears at least to explain the repulsion 
which exists in elastic, and, perhap. , other 
fluids. 
Before we quit (his subject, it will be worth 
while to shew, by an example, that the repul- 
sion between the particles of caloric often 
acts as a real chemical force, and that it af- 
fords a key to explain several phenomena 
which at first sight appear nearly contradic- 
tory. Why do bodies require different tem- 
peratures in order to unite ? and why does 
the presence of caloric in many cases favour, 
or rather produce, union, while it prevents 
or destroys it in others ? 
Some substances, phosphorus for instance, 
combine with oxygen at the common tem- 
perature of the atmosphere; others, as car- 
bon, require a higher temperature; and 
others, as hydrogen and azotic gas, do not 
combine, except at a very high temperature. 
To what are these differences ow ing ? 
It is evident, that whatever diminishes the 
cohesion which exists between the particles 
of any body, must tend to facilitate their che- 
mical union with the particles of other bo- 
dies. This is the reason that bodies combine 
more easily when held in solution by water, 
or when they have been previously reduced 
to a fine powder. Now caloric possesses the , 
property of diminishing cohesion : and on^ 
reason why some bodies require a high tem- 
perature to cause them to combine is, that at 
a low temperature the attraction of cohesion 
is in them superior to that of affinity; accord- 
ingly, it becomes necessary to weaken that 
attraction by caloric till it becomes inferior 
to that of affinity. The quantity of caloric 
necessary for this purpose must vary accord- 
ing to the strength of the cohesion and of the 
affinity; it must be inversely as the affinity, 
and directly as the cohesion. Wherefore, if 
w r e knew precisely the force of the cohesion 
between the particles of any body, and of the 
affinity between the particles of that body and 
of any other, we could easily reduce the tem- 
perature necessary to calculation. 
That caloric or temperature acts in this 
manner, cannot be doubted, if we consider 
that other methods of diminishing the attrae- 
