750 C O H E 
Confident; not contradictory to itfelf.—A coherent think¬ 
er, and a ftrift reafoner, is not to be made at once by a 
fet of rules. Watts. 
COHE'SION, f. [from cohere.~\ The aft of flicking to¬ 
gether.—Solids and fluids differ in the degree of cohefton , 
which, being increased, turns a fluid into a folid. Arbuth- 
not. —The ftate of union or infeparability: 
What caufe of their cohefion can you find ? 
What props fupport, what chains the fabric'bind? Blachn, 
Connection ; dependence.—In their tender years, ideas 
that have no natural cohefion come to be united in their 
heads. Locke. 
Cohefion is one of the four fpecies of attraction, denoting 
that force by which the parts of bodies adhere or (tick to¬ 
gether. This power was firft confidered by Newton as 
one of the properties effential to all matter, and the caufe 
of ail that variety obferved in the texture of different ter- 
reftrial -bodies. He did not, however, abfolutely deter¬ 
mine that the power of cohefion was an immaterial one; 
but that it might poflibly arife, as well as that of gravita¬ 
tion, from the aftion of another. Kis doClrine of cohefion 
Newt- n delivers in thefe words: The particles ofall hard 
homogeneous bodies, which touch one another, cohere 
with a great force ; to account for which, fome philofo¬ 
phers have recourfe to a kind of hooked atoms, which in 
effeft is nothing elfe hut to beg the thing in queftion. 
Others imagine that the particles of bodies are connected 
by reft, i. e. in effeft by nothing at all ; and others by 
confpiring motions, i. e. by a relative reft among them- 
felves. But it rather appears to us, that the particles of 
bodies cohere by an attractive force, whereby they tend 
mutually towards each other: which force, in the point 
of contact, is very great; at little diftances it is lels ; and 
at a little farther diftance quite infenfible. It is uncer¬ 
tain in what proportion this force decreafes as the diftance 
increafes. Pe aguliers conjectures, from fome pheno¬ 
mena, that; decreafes as the biquadratic or 4th power 
of the ciiltr fo that at twice the diftance it afts 16 
times me weakly, &c. Now if compound bodies be fo 
hard, as by experience we find fome of them to be, and 
yet have a great many hidden pores within them, and 
confift of parts only laid together; no doubt thofe Ample 
particles which have no pores within them, and which 
were never divided into parts, mult be vaftly harder. For 
fuch hard particles, gathered into a mafs, cannot poflibly 
v touch in more than a few points; and therefore .much 
lefs force is required to fever them, than to break a folid 
particle, whole parts touch throughout all their furfaces, 
without any intermediate pores or interftices. But how 
fuch hard particles, only laid together, and touching 
only in a few points, fhould come to cohere lo firmly, as 
in faft we find they do, is inconceivable; unlefs there be 
fome caufe, whereby they are attrafted and prelfed toge¬ 
ther. Now the Imalleft particles of.matter may cohere by 
the ftrongeft attractions, and conltitute larger, whofe at¬ 
tracting force is feebler : and again, many of thefe larger 
particles cohering, may conftitute others (till larger, 
whole attractive force is (till weaker, and fo on for feveral 
fucceflions, till the progreflion end in the biggeft particle, 
on which the operations in chemiftry, and the colours 
of natural bodies, do depend; and which by cohering 
compole bodies of a lenfible magnitude. 
1 he opinion maintained by many is that which is fo 
ftrongiy defended by J. Bernoulli, De Gravitate JEtheris ; 
who attributes the cohefion of the parts of matter to the 
uniform prelfure of the atmofphere; confirming this opi¬ 
nion by,the known experiment of two polilhed marble 
planes, which cohere very ftrongiy ip the open air, but 
ealily drop afunder in an exhaulted receiver. However, 
if two plates of this kind be fmeared with oil, to fill up 
the pores in their furfaces, and prevent the lodgment of 
air, and one of them be gently rubbed upon the other, 
they will adhere fo ftrongiy, even when fufpended in an 
exhaulled receiver, that the weight of the lower plate will 
SION. 
not be able to feparate it from the upper one. But, al¬ 
though this theory might fihve tolerably well to xplain 
the cohefion of Compolitions, or greater collections of 
matter, yet it falls far fhort of accounting for that firft 
cohefion of the atoms, or primitive corpulclds, of which 
the particles of hard bodies are compofed. 
Some philofophers have politively afferted, that the 
powers, or means,'are immaterial, by which matter co¬ 
heres ; and, in confequence of this fuppolition, they have 
fo'refined upon attractions and repuliions, that their fyf- 
tems feem but little fhort of fcepticifm, or denying the 
exiftence of matter altogether. A fyftem of this kind is 
adopted by Dr. Prieftley, from Meffrs. Bofcovich and 
Michell, to folve fome difficulties concerning the New¬ 
tonian doftrine of light. See his Hifrory of Vifion, vol. i. 
p. 392. “ The eafieft method,” fays he, “ of folving all 
difficulties, is to adopt the hypothelis of Mr. Bofcovich, 
who luppofes that matter is not impenetrable, as lias been 
perhaps univerfally taken for granted; but that it con- 
filts of phyfical points only, endued with powers of at¬ 
traction and repulfion in the fame.manner as folid matter 
is generally fuppofed to be: provided therefore that any 
body move with a fufficient degree of velocity, or have 
a fufficient momentum to overcome any powers of repul¬ 
fion that it may meet with, it will find no difficulty in 
making its way through any body whatever; for nothing 
elfe will penetrate one another but powers, fuch as we 
know do in faft exift in the fame place, and counterba¬ 
lance or over-rule one another. The moft obvious diffi¬ 
culty, and indeed almoft the only one, that attends this 
hypothefis, as it fuppofes the mutual penetrability of mat¬ 
ter, arifes from the idea of the nature, of matter, and the 
difficulty we meet with in attempting to force two bodies 
into the fame place. But it is demonftrable, that the 
firft obftruftion arifes from no actual contaft of matter, 
but from mere powers of repulfion. This difficulty we 
can overcome; and having got within one fphere of re¬ 
pulfion, we fancy that we are now impeded by the folid 
matter itfelf. But the very fame is the opinion of the 
generality of mankind with refpeft to the firft obftruftion,. 
Why, therefore, may not the next be only another fphere 
of repulfion, which may only require a greater force than 
we can apply to overcome it, without difordering the ar¬ 
rangement of the conftituent particles ; but which may¬ 
be overcome by a body moving with the amazing velo¬ 
city of light ? 
Other philofophers have fuppofed that the powers both 
of gravitation and cohefion are material; and that they 
are only different aftions of the etherial fluid, or elemen¬ 
tary (ire. In proof of this doftrine, they allege the ex¬ 
periment with the Magdeburg hemifpheres, as they are 
called. The prelfure of the atmofphere we lee is, in this 
cafe, capable of producing a very llrong cohefion ; and 
if there be in nature any fluid more penetrating, as well 
as more powerful in its effedls, than the air we breathe, 
it is poifible that what is called the attraftion of cohefion, 
may in lome meafure be an effeft of the aftion of that 
fluid. Such a fluid as this is the element of fire. Its ac¬ 
tivity is fuch as to penetrate all bodies whatever; and in 
the ftate in which it is commonly called fire, it afts 
according to the quantity of folid matter contained in 
the body. In this ftate, it is capable of diifolving the 
ftiongeft cohefions obferved in nature. Fire, therefore, 
being able to diffoh e cohefions, mult alfo be capable of 
caufmg them, provided its power be exerted for that pur- 
pofe, which poflibly it may be, when we confider its va¬ 
rious modes or appearances, viz. as fire or heat, in which 
ftate it confumes, deftroys, and diffolves; or as light,- 
when it feems deprived of that deltruftive power; and as 
the eleftric fluid, when it attrafts, repels, and moves 
bodies, in a great variety of ways. In the Philof.Tranf. 
for 1777, this hypothefis is noticed, and in fome meafure 
adopted, by Mr. Htnly. “ Some gentlemen,” lays he, 
“ have luppofed that the eledtric matter is the caufe of 
the cohefion of the particles of bodies. If the eleftric 
matter 
