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

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C H E M I
inity. five '7 snd tKe tefulfc of the combination of two power-
fully corrofive fubitances, is frequently a mild and in-
fipid compound.
It is indeed one of the chara&eriftics of chemical
affinity, that there be a total change in the proper¬
ties of the fubftances which enter into combination.
This change takes place in the fenfible qualities of
many of the compounds, and fome of thefe, as an illuf-
tration of this law, may be mentioned.
(i.) Changes of colour. The colour of lead is a
bluifh white, but when it combines with oxygen it af-
fumes a bright yellow or red colour, in proportion to
the quantity of oxygen. Cobalt, which is of a gray
colour, when combined with oxygen becomes of a
fine blue j and copper, which is red, combined in the
fame way, exhibits a green colour.
(2.) Changes in fmell. 1. The fmell of muriatic acid
is highly pungent •, ammonia, or the volatile alkali, is
not lefs fo j but when thefe two are combined, form¬
ing muriate of ammonia, or fal ammoniac, the new
compound is perfectly inodorous. This laft is a re¬
markable inftance of two highly volatile and odorous
fubftances becoming fixed in the compound, and defti-
tute of fmell, and thus exhibiting a total change of
their properties.
2. The fmell of fulphur and of potafti is fcarcely
perceptible in the uncombined ftate ; but when they
are united together, and moiftened with water, a moil
fetid and offenfive odour is emitted.
(3.) Changes in tafte. 1. The tafte »f fulphur is near¬
ly inlipid; and oxygen, which is one of the component
parts of the atmofphere, is not only innocent, but ne-
ceifary for the exiftence of animals : but w'hen thefe
two enter into union, the compound formed, which is
fulphuric acid, is one of the moil corrofive fubftances.
2. Sulphuric acicl, which is four and corrofive, forms
a combination with foda, which is alfo of a cerrofive
nature; the refult, which is Glauber fait, or fulphate
of foda, is a compound of a bitter naufeous tafte,
but pofleifing none of the properties of its Component
parts. /
Seventh Law.
The force of chemical affinity, is ejhmated by the force
which is neceffiary to feparate the fubjlances which enter
into combination.
In treating of cohcfton, or the attraction of Hggre-
gation, it wras ftated, that the method employed by
pliilofophers to eftimate that force, was to meafure the
oppofite force, or that which was neceflary to over¬
come the cohefive force. Thus, the w'eight attached
to the lower extremity of a metallic wire perpendicu¬
larly fufpended, which was juft fufficient to tear it
afunder, is confidered as the meafure of its power of
cohefion. But it will appear from what follows that
this law muft be adopted with confiderable modifica¬
tion.
Milieu 1° eftimating the force of chemical affinity, various
he time methods have been propofed by different philofophical
JiUtion; chemifts. It was thought by Wenzel, that the time
which one body required to diffolve another, might be
confidered as the meafure of the force of affinity be¬
tween thefe two bodies 5 but it muft appear from what
has been already faid, that the time of folution inuft
depend greatly on the cohefive force of the body
136
:e of
lity
l07
nated
S T R Y. . Ul
which is to be diffolved, and the nature of the com- AtHint)-. ,
pound which is formed j fo that from thefe deviations,
no certain meafure can be obtained from this method. I0g
According to fome, the meafure of the force of che- Ijv th dif-
mical affinity may be eftimated by the difficulty of fG-fi 'u^tv °*
parating the fubftances which have entered into com-tcPai:atl''n*
bination 5 or, by taking the compound ratio of the fa¬
cility with w'hich they are combined. But as no me¬
thod has been invented to afeertain either the one or
the other, which are the neceflary previous fteps iq
the method propofed, it is impoffible, in this way, to
eftimate the force of chemical affinities. 10$
Obferving the effects of the union and abftraftion of by the af-
caloric, in the operations of chemical affinity, Lavoi-^cr
fier and La Place, in a memoir publiffied in 1783,''“
propofed this as the method of eftimating the force of
affinity. But it feems fcarcely poflible to meafure the
force of chemical affinity between two fubftances by
the degree of temperature which is required to over¬
come the force of cohefion j or, as this degree of tem¬
perature has no meafurable proportion with the force
of chemical affinity, it can afford no data for eftima¬
ting this force. And this quantity being variable and
unknown, a fixed term is wanting to form a fcale of
comparifon.
We have already mentioned, in treating of adliefioq,
the experiments of Dr Taylor on the adhefion of fur-
faces, and the experiments and concluftons of Morveau
and Achard on the fame fubjeef. From thefe Mor¬
veau has propofed to deduce a method of eftimating
the force of chemical affinities. But for an account
of this, we refer the reader to the firft fe£tion. IIC
A different method has been propofed by Mr Kir- KirwanV
v’an, in his experiments and oblervations on the
tractive powers of mineral acids*. He obferves,voit
that the principal end which he had “ in viewr was, ixxifi,
to afeertain and meafure the degrees of affinity or
attraction that fubfift between the mineral acids, and
the various bafes with which they may be combined j
a fubjeCt of the greateft importance, as it is upon
this foundation that chemiftry, confidered as a fciencc,
muft finally reft j and though much has been already
done, and many general obfervations laid down on
this head, yet fo many exceptions have occurred, even
to fuch of thefe obfervations as feem to have been moft
firmly eftabliflied, that not only a variety of tables of
affinity have been formed, but many very eminent
chemiils have been induced to doubt whether any gene¬
ral law w hatfoever could be traced f.” f/£j& p.341
u The difeovery of the quantity of real acid in each
of the mineral acid liquor#, and the proportion of
real acid taken up by a given quantity of each bafis
at the point of faturation, led me unexpectedly to what
feems to me the true method of inveftigating the quan¬
tity of attraction which each acid bears to the feveral
bales to which it is capable of uniting. For it was
impoffible not to perceive,
lit, That the quantity of real acid neceffary to fa-
turate a given weight of each bafe, is inverfely as the
affinity of each bafe to filch acid.
2dlv, That the quantity of each bafe requifite to
faturate a given quantity of each acid, is direCtly as- _ ,
the affinity of each acid to fuch bafe.
Thus, 100 grs. of each of the acids require for their
faturation, a greater quantity of fixed alkali than, of
3 K 2 calcareous