C O H 
C O H 
75i 
matter be, as I fufpefl, a real elementary fire inherent lit 
all bodies, that opinion may probably be well founded ; 
and perhaps the foldering of metals, and the cementation 
of iron, by fire, may be confidered as ftrong proofs of 
the truth of their hypothefis.” 
But whatever the caufe of cohefion may be, its effects 
are evident and certain. The different degrees of it con- 
ilitute bodies of different forms and properties. Thus, 
Newton obferves, the particles of fluids, which do not 
cohere too ftron’gly, and are fmajl enough to render them 
fufceptibie of thofe agitations which keep liquors in a 
fluor, are molt ealily feparated and rarefied into vapour, 
and make what the chemilts call volatile bodies ; being rare¬ 
fied with an ealy heat, and again condenfed with a mode¬ 
rate cold. Thofe that have grofler particles, and fo are lefs 
fufceptibie of agitation, or cohere by a ftronger attra&ion, 
are not feparable without a greater degree of heat, and 
l'ome of them not without fermentation: and thele make 
what the chemifts call fixed bodies. Air, in its fixed Hate, 
polfeffes the interftices of folid fubftances, and probably 
fierves as a bond of union to their conftituent parts; for 
when thele parts are feparated, the air is difcharged, and 
recovers its elafticity. And this kind of attraction is 
evinced by a variety of .familiar experiments ; as, by the 
union of two contiguous drops of mercury; by the mu¬ 
tual approach of two pieces of cork, floating near each 
other in a bafon of water; by the adhefion of two leaden 
balls, whofe furfaces are fcraped and joined together with 
a gentle twill, which is fo conliderable, that, if the fur- 
faces are about a quarter of an inch in diameter, they 
will not be feparated by a weight of ioolb; by the afcent 
of oil or water between two glafs planes, fo as to form 
the hyperbolic curve, when they are made to touch on 
one fide, and kept feparate at a fmall diftance on the 
other; by the depreffion of mercury, and by the rife of 
water in capillary tubes, and on the fides of glafs vefl'els; 
alfo in fugar, fponge, and all porous fubftances. And 
where this coheftve attraction ends, a power of repui- 
fion begins. 
To determine the force of cohefion, in a variety of dif¬ 
ferent fubftances, many experiments have been made, and 
particularly by profeflor Mufchenbrcek. The adhefion 
of polifhed planes, about two inches in diameter, heated 
in boiling water, and fineared with greafe, required the 
following weights to feparate them : 
Cold greafe. Hot greafe. 
Planes of Glafs - - - 1301b. - - - 3001b. 
Brafs - --130--- - 800 
Copper - - 200 - - - - 8jo 
Marble - - 225 - 600 
Silver - --150--- - 250 
Iron - - 300 - - - - 950 
But when the brafs planes were made to adhere by 
other forts of matter, the relults were as in the following 
•table: 
With Water -------- 12 oz. 
Oil. 18 
Venice Turpentine - - - - 24 
Tallow Candle ----- 800 
Rofin - -- -- -- - 830 
Pitch - -- -- -- - 1400 
In eftimating the abfolute cohefion of folid pieces 
©f bodies, he applied weights to feparate them accord¬ 
ing to their length : his pieces of wood were long 
fiquare parallelopipedons, each fide of which was ‘26 of 
an inch, and they were drawn afunder by the following 
weights: 
Fir - -- -- -- -- - goo lb. 
'Elm - -- -- -- -- - 930 
Alder ---------- 1000 
Linden-tree - -- -- -- - sooo 
Oak - -- -- -- -- -1150 
-Beech - -- -- -- -- - 1250 
Alh - -- 1250 
He tried alfo wires of metal, i-ioth of a Rhinland inch 
in diameter; the metals and weights were as follow : 
Of Lead --------- - 29^ lb< 
Tin - - ,.4oi 
Copper - - - -.299^ 
Yellow brafs ------- 360 
Silver - -- -- -- -- 370 
Iron - -- -- -- -- - 430 
Gold - -- -- -- -- - 300 
He then tried the relative cohefion, or the force with 
which bodies refilt an aCtion applied to them in a direct 
tion perpendicular to their length. For this purpofe he: 
fixed pieces of wood by one end into a lquaie hole in a 
metal plate, and hung weights towards the other end, till 
they broke at the hole; the weights and diftances from 
the hole are exhibited in the following table : 
Diftance. Weight. 
Pine t - - 9A inch. 36^ oz. 
Fir .--9 ------- 40 
Beech - -7 - -- -- -- 36-J. 
Elm ' - - - 9 44 
Oak - - - $$ ------- 4 g 
Alder - - - 94 ------- 48 
COHE'SIVE, adj. That has the power of iticking to 
another, and of refilling feparation. 
COHE'SIVENESS,/. The quality of being cohefive 3 
the quality of refilling feparation. 
To COHPBIT, <v.a, [cohibeo , Lat.] To reftrain 5 to 
hinder. 
To CO'HOBATE, v. a. To pour the diftilled liquor 
upon the remaining matter, and diltil it again.—The 
juices, of an animal body are, as it were, cohobated, being 
excreted, and admitted again into the blood with the 
frelh aliment. Arbuthnot. 
COFIOBA'TION,/. A returning any diftilled liquor 
again upon what it was drawn from, or upon frelh ingre¬ 
dients of the fame kind, to have it the more impregnated 
with their virtues. Suincy.—Cohobation is the pouring the 
liquor diftilled from any thing back upon the remaining 
matter, and diltilling it again. Locke. 
It is frequently required in chemical operations, that 
a fubftance lhould be for a long time boiled, or digelted 
in a volatile fluid confiderably heated. This neceil’arily 
occafions a great lofs of the fluid, if the operation be per¬ 
formed in an open velfel. To prevent this, the ancient 
chemifts employed a kind of ftill, called a pelican. The 
head of this veflel was provided with two beaks, which' 
by a gradual incurvation returned into the body, and 
conveyed thither whatever volatile matter had rilen into 
the head, and there became condenfed. The views and 
practices of the modem chemifts do not require thele long- 
digeftions; but many valuable experiments of Priellley 
and others, of the nature of repeated diftillation, have 
been made by a Ampler apparatus. A glafs tube, three 
or four feet in length, lupplies the place of the pelican. 
A mineral acid water, or any other requilite fluid, is put 
into the tube, in fuch a quantity as to occupy an inch or 
more of its lower end; the upper end is then hermeti¬ 
cally feale<l, and heat applied to the lower by a land-bath. 
The fluid partly riles, and is partly changed, in other 
refpefls, by the heat. The vapours, as they arrive towards 
the upper part of the tube, become condenfed, and return 
again to the lower in a kind of circulation down the cooler 
fide of the veflel. See the various experiments in the 
article Chemistry in this volume. 
COHO'EZ, or the Falls in Mohawk river, between 
two and three miles from its mouth, and ten miles north¬ 
ward of Albany, are a very great natural curiofity. The 
river above the falls is about three hundred yards'wide, 
and approaches them from the north-weft in a rapid cur¬ 
rent, between high banks on each fide, and pours the 
whole body of its water over a perpendicular rock of 
about fifty feet in height, which extends quite acrofs like 
a mill-dam. The banks of the river, immediately below 
the 
