Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 19, Scripture-SUG

S P E [ 564 1 s P E
Specific fluids, examined by the hydroflatical balance of Mr
Gravity. g0yje> j)r Hooke made a prodigious number, cbief-
ly on articles of commerce, which were unfortunately
loft in the fire of London.
It was fopn found, however, that Lord Bacon’s con-
jedture had been well founded, and that bodies changed
their duifity very fenfibly in many cafes. In general,
it was found that bodies which had a ftrong chemical
affinity increafed in denfity, and that their admixture
was accompanied with heat.
By tins difcoverv it is manifeft that Archimedes had
not folved the problem of detedfing the quantity of fil-
Vt. r mix d with the gold in King Hiero’s crown, and
that the phyiical folution of it requires experiments made
on all the kinds of matter that are mixed together.
We do not find that this has been done to this day, aL
though we mav aftirm that there are few queftions of
more importance. It is a very curious fadl in chemi-
ftrv, and it would be moft defirable to be able to re¬
duce it to fome general law's : l1 or inftance, to aicertain
what is the proportion of two ingredients which pro¬
duces the greateft change of denfity. This is impor¬
tant in the fcience of phyfics, becaufe it give us confi-
derable information as to the mode of adtion of thofe
natural powers or forces by which the particles of tan¬
gible matter are united. If this introfufeeption, con¬
centration, compenetration, or by whatever name it
be called, were a mere reception of . the particles of
©ne fubitance into the interftices of thofe of another,
it is evident that the greateft concentration would be
obferved when a fmall quantity of the recipiend is mix¬
ed with, or diffeminated through, a great quantity of
the other. It is thus that a fmall quantity of fine land
will be received into the interftices of a quantity of
fmall fhot, and will increafe the weight of the bagful
without increafing its bulk. T he cafe is nowftfe differ¬
ent when a piece of freeftone has grown heavier by
imbibing or abfnrbing a quantity of water. If more
than a certain quantity of fand has been added to the
fmall (hot, it is no longer concealed. In like manner,
various quantities of water may combine with a mafs of
clay, and increafe its fize and weight alike. All this
is very conceivable, occafioning no difficulty.
But this is not the cafe in any of the mixtures we
are now confidering. In all tbefe, the firft additions of
either of the two fubftanees produce but an inconfider-
able change of general denfity ; and it is in general
moft remarkable, whether it be condenfation or rare-
fa&ion, when the twm ingredients are nearly of equal
bulks. We can illuftrate even this difference, by re-
flefting on the imbibition of w-ater by vegetable folids,
fuch as timber. Some kinds of wood have their weight
much more increafed than their bulks •, other kinds of
wood are more enlarged in bulk than in weight. The
like happens in grains. This is curious, and ffiows in the
moft unqueftionable manner that the particles of bodies
are not in contadi, but are kept together by forces
which acl at a diftance. For this diftance between
the centres of the particles is moft evidently fufceptible
of variation •, and this variation is occafioned by the in-
troduclion of another fubftance, which, by adiing on
the particles by attradlion or repulfion, diminiffies or
ihcreafes their mutual actions, and makes new diftances
neceflary for bringing all things again into equilibrium.
We refer the curious reader to.the ingenious theory of.
4
the abbe Bofcovich for an excellent illuftration of this
fubjedl (Theor. Vhil. Nat. § do Solutione Chemica.)
This queftion is no lefs important to the man of bu-
finefs. Till we know the condenfation of thofe metals
by mixture, we cannot tell the quantity of alloy in gold
and filver by means of their fpecific gravity nor can we
tell the quantity of pure alcohol in any fpirituous liquor,
or that of the valuable fait in any lolution of it. For
want of this knowledge, the dealers in gold and filver
are obliged to have recourfe to the tedious and difficult
teft of the affay, which cannot be made in all places or
by all men. It is therefore much to be wilhed, that
fome perfons would inftitute a feries of experiments in
the moft interefting cafes : for it muft be obferved, that
this change of denfity is not alway s a fmall matter *, it is
fometimes very confiderable and paradoxical. A re¬
markable inftance may be given of it in the mixture of
brafs and tin for bells, great guns, optical fpeculums,
&.c. The fpecific gravity of call brafs is nearly 8.006,
and that of tin is nearly 7.363. If two parts of brafs be
mixed with one of tin, the ipecific gravity is 8.917 \
whereas, if each had retained its former bulk, the fp. grav.
( 2 X 8.006 ft-7-363
would have been only 7-793
(=
Spedfit
Gravity.
)•
A mixture of equal parts flrould have the fpecific gravity
7.684 '■> but it is 8.441. A mixture of two parts tin
with one part brafs, inftead of being 7.577, is 8.027.
In all thefe cafes there is a great increafe of fpecific
gravity, and confequently a great condenfation of parts
or contraflion of bulk. The firft mixture of eight cu¬
bic inches of brafs, for inftance, with four cubic inches
of tin, does not produce 12 cubic inches of bell-metal,
but only 10ft nearly, having ffirunk It would ap¬
pear that the diftances of the brafs particles are moll af-
feifted, or perhaps it is the brafs that receives the tin in¬
to its pores •, for w'e find that the condenfations in thefe
mixtures are nearly proportional to the quantities of the
brafs in the mixtures. It is remarkable that this mix¬
ture with the lighteft of all metals has made a compofi-
tion more heavy and denfe than brafs can be made by
any hammering.
The moft remarkable inftance occurs in mixing iron
with platina. If ten cubic inches of iron are mixed with
of platina, the bulk of the compound is only 9ft
inches. The iron therefore has not limply received the
platina into its pores : its own particles are brought
nearer together. There are fimilar refults in the folu¬
tion of turbith mineral, and of fome other falls, in wa¬
ter. The water, inftead of riling in the neck of the vef-
fel, when a fmall quantity of the fait has been added to
it, finks confiderably, and the two ingredients occupy
lefs room than the water did alone.
The fame thing happens in the mixture of water with
other fluids and different fluids with each other : But u'e
are not able to trace any general rule that is obferved
with abfolute precifion. In moft cafes of fluids the
greateft condeniation happens when the bulks of the in¬
gredients are nearly equal. Thus, in the mixture of al¬
cohol and water, we have the greateft condenfation
when 16I ounces of alcohol are mixed with 20 ounces
of water, and the condenfation is about of the whole
bulk of the ingredients. It is extremely various in dif¬
ferent fubftanees, and no claffification of them can be
made in this refpe£t.
A diflertation has been publifhed on- this fubjeft by