CHEMISTRY. 
a medium. Thus oil and water will not 
unite, but either of them will unite with an 
alkali i and if this last be united with oil, 
it forms soap, which can be united with wa- 
ter. The alkali is the medium ; or (c) two 
principles which attract each other may 
neither of them be capable of direct or 
ready union with a third ; but when the 
two former shall be actually combined 
together, the compound shall attract and 
combine with the third body and form a 
triple compound. This new power is called 
resulting attraction. Thus neither sulphur 
nor potash have any sensible action upon 
gold ; but when tliey are fused together 
they combine with that metal. Most of 
the effects of resulting attraction are conse- 
quences of the change of state of bodies, 
particularly to that of fluidity ; and the ef- 
fects of this attraction and that by a me- 
dium often exist in the same case ; or, (/) 
if we suppose three principles to be in 
such circumstances of aggregation or tem- 
perature as to have no perceptible dispo- 
sition to unite in pairs, but that the result- 
ing attraction of a compound of two of 
tlicra, if united, would then act upon 
the third, and produce a triple combina- 
tion, it may happen that this resulting at- 
traction, which seems to be only in pi-os- 
pcct, shall have pow'er to complete the 
triple compound ; and the modification is 
called disposing attraction. Thus vinegar 
has no perceptible action upon copper, but 
it can dissolve the compound of copper and 
oxygen, called the oxide of copper : nei- 
ther vinegar nor copper have any disposi- 
tion to take oxygen from its elastic state in 
the atmosphere, so that copper and vinegar 
may be kept together without solution in a 
closed vessel : but if the air be admitted, 
the presence of the vinegar will dispose the 
copper to take oxygen and form an oxide, 
and with this combination tlie vinegar will 
unite. There is much convenience in the 
teiin, disposing attraction, as used to ex- 
press this phenomenon, though it must be 
confessed that this prospective disposition, 
ascribed to unconscious beings, seems to 
produce some confusion in the mind. It 
may therefore be proper to notice that the 
case seems to belong to disposing attrac- 
tion, and may be thus hypothetically ex- 
plained. Copper, and several other metals 
which attract oxygen from the air, become 
covered with a tliin oxide or rust, which 
prevents any farther access of that fluid, and 
consequently it rusts no fai'ther, unless the 
thin coat of oxide be scraped off and a 
new surface exposed, and if this were con- 
turned to be done, all the copper would be 
gradually oxided. Now the vinegar, by 
, the condition of our case, does this, and the 
copper is gradually and totally • dissolved ; 
not, as it appears, because the copper and 
oxygen are disposed to unite by a third 
power, which, as it were, waits tor them, 
but because this power removes an impedi- 
ment which w'ould impede their progressive 
union. 
(g) The case of attraction, which has 
most particularly engaged the attention of 
chemistry, is that where two principles be- 
ing combined are separated from each other 
by the addition of a third, which combines 
with one of them. This has, perhaps im- 
properly, been called simple elective attrac- 
tion, and by others precipitating attraetion : 
its principal effects or distinguishing charac- 
ter would, it seems, be better designated by 
the terms decomposing attraction. Thus, 
if sulphuric acid and magnesia be combined 
in the salt called sulphate of magnesia (dis- 
solved in water) and potash be added, the 
acid will unite with this last, and the mag- 
nesia will be separated and fall down. It 
was for a long time thought that these com- 
binations and separations were complete 
and entire ; but they appear in every in- 
stance to form cases of the intermediate or 
resulting attractions, wherein the propor- 
tions of the soluble and insoluble parts are 
extremely different, and the degrees of 
saturation often modify the results. For the 
body separated has always a small propor- 
tion of the two others, and the new com- 
pound usually suspended is not binary, but 
triple at least ; and the proportions and 
effects ai e more dr less altered by the quan- 
tity of solvent present, and the aptitude of 
the new combinations to take the solid, 
fluid, or elastic states. 
Tables of separation or decomposition 
have been called tables of simple elective 
attraction. They are usually drawn up to 
express effects in the humid way with mo- 
derate proportions of water at the middle 
atmospheric temperature ; and in the dry 
way by the operation of fire acting upon 
the containing vessels to raise their tempe- 
rature. From the reasons just mentioned, 
they cannot be admitted as denoting inva- 
riable effects, but they are nevertheless 
useful, provided the modifications of cir- 
cumstances be attended to in our general 
reasoning. See Elective Attkaction. 
3. In our present discussion it was not 
practicable, from the nature of tlie subject. 
