Tamarind 
— 
. — 
8,750 
Fir 
— ■ 
. — . 
8,330 
Walnut 
• — 
— 
8,130 
Fitch-pine 
— . 
— 
7 ,656 
Quince 
— 
— 
6,750 
Cypress 
— 
— 
6,000 
Poplar 
— 
— 
5,500 
Cedar 
— 
— 
4,880. 
3. The nature of cohesion has been more 
[happily explained by Boscovich than by any 
[other philosopher. Indeed it forms the most 
[beautiful and satisfactory part' of his theory. 
[According to him, the particles of bodies co- 
here together when they are placed in the 
limit of repulsion and attraction. Two par- 
‘ tides, when situated at a certain distance 
from each other, repel each other mutually ; 
this repulsion gradually diminishes as the dis- 
tance between the particles increases, till at 
last when the distance reaches a certain mag- 
nitude, the repulsion ceases altogether. If the 
[distance is increased ever so little, the par- 
ticles now, instead of repelling, attract each 
: other; and this attraction increases with the 
distance till at last it reaches its maximum. 
From this point it gradually diminishes, till 
at last, when the particles have acquired a 
certain distance, it vanishes altogether. If 
’the distance is increased ever so" little be- 
jyond that distance, the particles now again 
repel each other. He supposes that the "’in- 
sensible distance between two particles is 
divided into an indefinite number of por- 
tions of alternate repulsions and attractions. 
Boscovich supposes, that in all cases of 
[cohesion the particles of the cohering body 
|are so situated as to be in these limits of co- 
hesion with respect to each other. Accord- 
ing to this very ingenious theory, cohesion is 
not, properly speaking, a force, but the in- 
terval between two forces. And even if 
we were to modify the theory a little, still 
\ve must consider cohesion as the balancing 
of two opposite forces, either of which be- 
comes prevalent according as the cohering 
particles are urged nearer each other or forced 
to a greater distance. Consequently if we 
were to speak with precision, cohesion is not 
! itself a force, but the absence of a force. 
I What has been hitherto called the force of 
'cohesion, is the attraction which prevents the 
cohering particles from separating from each 
(other, and which begins to act, or more 
precisely, which becomes prevalent, when the 
, particles- are urged to a greater distance from 
each other. 
4. Boscovich has shewn, in a very satis- 
factory manner, how all the varieties of co- 
, hesion may be produced by the differences 
in the size, figure, and density, of the cohe- 
ring particles. It deserves attention, that in 
most cases the cohesive force of simple bo- 
dies is greater than that of compound bodies. 
o this indeed there are a great number 
of exceptions, but the observation holds in a 
variety of instances. Ail the metals cohere 
very strongly ; the diamond probably co- 
| he res with no less force, if we can judge from 
jits hardness ; and the cohesion of sulphur is 
also very considerable. Thus if we except 
phosphorus, all the simple substances are re- 
pnarkable for cohesion. Those of them 
which are in the state of elastic fluids must 
be excluded altogether; because in that 
particular state the particles, instead of being 
Po ^ ie limit of cohesion, are actually repelled" 
COHESION. 
In the earths, too, such of them at least are 
foil nil crystallized in a, state of purity, the < o- 
hesiou is very strong. Thus the sapphire 
or crystallized alumina, and rock crystal or 
crystallized silica, are always very hard, and 
exhibit a much stronger cohesion than lime- 
stone, or magnesian stones, which are com- 
posed or heterogeneous bodies. This re- 
mark, however, by no means applies to the 
metals ; tor in them the cohesion is very of- 
ten increased considerably by alloying them 
together. Thus the cohesion of copper is 
doubled by alloying with one-sixth of its 
weight of tin, though the cohesion of the tin 
is scarcely one-sixth of that of the copper. 
1 he cohesion of metals is greatly increased 
by forging them, and by drawing them out 
into wire. By this last operation gold, silver, 
and brass, have their cohesion nearly tripled ; 
copper and iron more than doubled. 
5. There are three states in which bodies 
exist exceedingly distinct from each other : the 
state of solids, or liquids, and of elastic fluids. 
In the first two states the particles cohere 
with more or less force ; but the cohesion 
produces in them very different effects. In 
the first it prevents all relative motion 
among the particles themselves ; in the se- 
cond, this relative motion is left at full liberty. 
Hence in solid bodies the motion of one par- 
ticle is followed by the motion of the whole 
mass ; or if that is impossible, the cohesion is 
destroyed altogether. In liquids, on the con- 
trary, the motion of one particle is not ne- 
cessarily followed by that of the rest, neither 
does that motion destroy the cohesion. Bos- 
covichhas shewn, that solidity and fluidity are 
the consequence of the f igure of the cohering 
particles. If that figure is such that the par- 
ticles may change their position without al- 
tering their relative distances, the consequence 
must be fluidity ; because in that case there 
is nothing to oppose the motion of any in- 
dividual particle. This happens when the 
particles are spherical ; but if the figure is 
such that the particles cannot change their 
position without altering their relative dis- 
tances, the bodies which they compose must 
be solids, because all relative motion of an 
individual particle is .opposed by the at- 
tractions and repulsions of all the sur- 
rounding particles; for every motion must 
bring the particle out of the former limit of 
cohesion. This happens when the particles 
have the figure of parallelopipeds, or any 
other figure except that of spheres. 
This explanation is exceedingly ingenious ; 
but it would not be an easy task to explain by 
means of it all the phenomena of solidity and 
fluidity. How comes it, for instance, that the 
addition of a certain dose of caloric renders a 
body fluid which was before solid ? If it be 
answered, that it acts by combining with the 
particles of the solid in such a manner as to 
render them spherical ; how comes it, in that 
case, that gold and platina, metals which 
are ductile and malleable, properties which 
indicate a kind of approach to fluidity, and 
of course sphericity, in the particles of’these 
metals, — how comes it that they require so 
much more caloric to render them fluid than 
bismuth or sulphur, which are altogether 
brittle ? We must rather consider fluidity as 
a kind of solution in caloric, analogous to the 
solution of salts in water. But this explana- 
tion, though it would do very well in many 
instances, would lead us in others to difficul- 
383 
Fes as great as those which we are endeavour- 
ing to avoid. 
The cohesion of liquids is often very con- 
siderable. According to sir , Isaac Newton, 
it is nearly proportional to the density of the 
liquid. 1 his holds pretty accurately in se- 
veral instances : but it is not easy to ascer- 
tain (he cohesion of a liquid with precision, 
because the particles slide upon each other ; 
and the column of the liquid, whose cohesion 
we are measuring, always becomes smaller 
and smaller, till at last it consists only of a 
very small number of particles. 
Viscid bodies have particles approaching to- 
a spherical fo m ; but deviating from it so 
far as to occasion a certain resistance to the 
relative motion of the particles. 
Solid bodies are of two kinds: they may 
either resist all change of distance in their 
particles so strongly as not to be capable of 
compression or dilatation without a breach of 
cohesion ; or they may admit of both to a 
certain degree with facility. 1 he first of 
these constitutes hardness ; the second con- 
stitutes softness if the' particles retain their 
new situation, or elasticity if they return 
again to their old position when the external 
force is removed. Ductility and malleability 
depend upon the same state as softness, only 
the particles require a greater force to make 
them change their situation, and assume a 
new' one. 
6. When a solid body is plunged into a li- 
quid, if the particles of the liquid attract 
those of the solid with a greater force than 
these last particles attract each other, they 
are gradually carried off by the fluid, and 
combine with its particles, that is to say, the 
solid is gradually dissolved. Thus sugar is 
dissolved by water, and sulphur by oil. The 
particles of the solid thus dissolved are each 
of them surrounded and combined with a 
certain number of the particles of the liquid. 
Hence they must be arranged in the liquid 
in regular order, and at regular distances 
from eacli other. The greater the number of 
particles thus dissolved bv the liquid, the 
smaller is the affinity by which each of them 
is retained, because it is surrounded by 
a smaller number of particles of the li- 
quid. But the greater must be the force 
with which these particles are attracted to- 
wards each other, and of course tend to form 
the solid again by cohesion ; because the 
greater the number of the particles of the- 
solid dissolved in the fluid, the nearer they 
are to each other. 
Thus it appears that the affinity between the 
fluid and solid diminishes with the quantity 
dissolved ; but that the tendency to cohesion 
increases with that quantity. Consequently 
if the solution is supposed to go on, these 
two opposite forces must at last balance one 
another ; and whenever that happens, the 
liquid can dissolve no more of the solid. If 
it did, the particles of the solid would in 
part cohere, and form a new portion of the 
solid again. Whenever this happens, the fluid 
is said to be saturated. The saturation of a 
fluid then does not mean that its affinity for 
the solid is satisfied, but that it is not greater 
than the tendency of the combined particles 
to cohere. Now when a liquid is saturated 
with a solid, if by any means we can abstract 
part of that liquid, the cohesive force of the 
particles of the soiid must gain the superi- 
ority; and the consequence will be, that they. 
