Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 5, BUR-CHI

, 57.
All budies
attradl.
432 C H E M
, AiIin!ty- afnnities, which has led to the explanation of a great
number of apparent anomalies. Since the time of
Bergman, tins fubject has been profecuted by many of
the moif diftinguilhed philofophical ehemifts. Among
thefe we may mention the induftrious and indefatigable
Kirwan of our own country 3 and among the French
philofophers, Morveau, and more efpecially Berthollet,
diitinguilhed for his fkill and fagacity, who has lately,
in his refearches concerning the laws of affinity, opened
a new field of enquiry, correfted many former errors,
and pointed out fome new laws in this interefting and
important fubjeft.
All bodies with which we are acquainted, are influ¬
enced by a certain force, by which they are attrafted,
or drawn towards each other. A done, when it is
unlupported, falls to the ground 3 the planets are at¬
tracted by the fun 3 two polithed plates of metal, of
glafs, or of marble, when brought into clofe contaft,
adhere w ith a certain force 3 a piece of wood or done
requires a condderable degree of force to feparate the
particles, or to draw it afunder 3 and lime and fulphu-
ric acid enter into fuch clofe combination, that it re¬
quires an equal degree of force to overcome that com¬
bination, or to feparate the particles from each other.
Whatever may be the nature of thefe attractions, or
the caufe of thefe different combinations, or whether
they are to be aferibed to the fame univerfal law per¬
vading matter, tis fome have fuppofed, they have been
deferibed by philofophers under different names. The
attraction which exifls between all bodies in the folar
fydem, was denominated by Newton, by the general
term attraBion; and he demondrated that this uniform
and univerfal law' w'as precifely the fame as the lavr of
gravitation, or the defeent of heavy bodies towards the
earth 3 and that this attraction was an effential proper¬
ty of all matter 3 that the minuted particles, in propor¬
tion to their bulk, w'ere equally influenced with the
larged maffes 3 that the fame power which retained the
planets in their orbits, gave form to the drops of rain.
We have faid, that thefe different forces or. attrac¬
tions have been didinguiffied by different names. That
attraction which is exerted between tw'o poliffied fur-
faces brought into contaCt, has been called adhejion.
When particles of the fame nature are attraCled or held
together, the expreffion of the force by which this is
effeCted, has received the name of cohejion, homogene¬
ous affinity, or the attraBion of aggregation ; but when
diffimilar particles, or the particles of twm fubdances
of different qualities, combine together, the force or at¬
traction which is here exerted has been called heteroge¬
neous affinity, the attraBion of compofition, or, driCtly
fpeaking, chemical affinity. In the three following fec-
tions, wre propofe to give an account of the opinions and
doCtrines which have been held by philofophers with
regard to the nature and force of thefe attractions. Of
the two fird we lhall only take a fhort view' 3 but fliall
enter more fully into the detail of the latter, namely,
chemical affinity, which more driCtly belongs to our
prefent fubjeCt.
Sect. I. Of Adhesion.
By adheffon, then, is to be underdood, that force
which retains different fubdances in contaCt with each
other. Thus, water adheres to the finger, which is
5.8
Different
names of
affinity.
I S T II Y.
faid to be wet, and mercury brought into contaCl w ith Affin
gold, adheres w ith great force. Adhefion takes place, y
either between tw 0 folids, as marble or glafs 3 or be- 51
tween folids and fluids, as when water riles in capilla- ^n w!l ij
ry tubes 5 or between two fluids, as v/ater and oil, InCesiti
an experiment made by Dr Defaguliers, he oblerved, pens,
that two plates of glafs, of one-tenth of an inch in dia¬
meter, adhered with a force equal to 17 ounces. The
adhefion of two fluids has been proved by the experi¬
ment of Lagrange and Cigna, as that of oil and w ater,
between which it was formerly fuppofed there exided
a natural repulfion 3 and the experiments on capillary
attraction, and particularly the afeent of water between
two panes of glafs, which was afeertained by Dr Brook
Taylor, have ellablilhed the attraction between folids
and fluids.
This adhefive force, or the caufe of this attraction, ^cco^
has been differently accounted for by philofophers. In for.
a differtation on the w eight of the atmofphere, pub-
lilhed in 1682 by James Bernoulli, he aferibes the
refidance which two poliflied pieces of marble oppof-
ed to their feparation to the preflure of the air 3 and
in proof of this, he dates as a faCt, that the two plates
could be eaiily feparated in vacuo. But it has been
fuppofed that he had either never attempted to verify
this faCt, or that the experiment had been accompanied
by fame fallacy. From the experiments made by Dr
Taylor, he concluded that the intenfity of the adhefive
power of furfaces might be meafured by the weight
which was required to feparate them. About the fame
time Mr Haw kfbee proved by experiment, that the aff-
hefion of furfaces and capillary attraction were not to
be aferibed to the preffure of the atmofphere, as Ber¬
noulli had fuppofed 3 but Lagrange and Cigna, after
having proved the adhefion between oil and water,
thought that it was owing to a different caufe from
that of attraction. They fuppofed that it was occafioned
by the preflure of the air, and that the opinion of Dr
laylor w as not well founded. Such were the opinions
held by philofophers on this fubjeCt, when Morveau, in
the year I773> was to inftitute a feries of experi¬
ments on adheiion, which he exhibited at Dijon. By
thefe experiments he proved, that this attraction was
not owing to the preflure of the air, but entirely to
the attraction of the two fubftances .between them-
felves. I o prove this, a poliflied plate of glafs was
fufpended from the arm of a balance, and placed in
contaCt with a furface of mercury. The weight ne-
eefiary to feparate the two furfaces was equal to nine
gros and fome grains. The whole apparatus was pla¬
ced under the receiver of an air-pump, which was ex-
haulted of the air as much as poffible. It required
exaCtly the fame force to feparate the furfaces. The
lame dilk of glafs brought into contaCt with pure wa¬
ter, adhered to it W’ith a force equal to 258 grains; hut
from the furface of a folution of potalh, it required only
a force of 210 grains. This inequality of effeCts with
equal diameters, and in the inverfe order of the re-
fpeCtive denfities, feemed not only to be detifive in
favour of Dr Taylor’s method, but appeared to point
out the poffibility of applying it to the calculation of
chemical affinities. For the force of adhefion being
neceffarily proportional to the points of contaCl, and
the fum of the points of contaCt not varying in the
adhefion of a fluid and a folid with equal lurfaces, but
by