279
DOUBLE EXCHANGES.
or converts them into vapour. (See Note 22. at the end of the
Volume.)
Upon the whole, Mr. Geoffroy’s table, considered as the first
thought, and first attempt in this way, is highly worthy of praise.
But such a multitude of discoveries in chemistry have been made
fince his time, that his original table gives now but a narrow and im
perfect view of the numerous facts which a table of this kind can be
made to represent and suggest. Accordingly, several improved and
more comprehensive tables of elective attractions and exchanges have
been drawn up by different chemists ; but none has employed so
much pains on this subject, or given such a comprehensive and co
pious table, as the late Professor Bergmann of Sweden, whose works
on this and other parts of chemistry are translated into our language.
And fince that time, Mr. Morveau in the New Encyclopædia. (See
Note 23. at the end of the Volume.)
Among the most remarkable additions which have been made to
these tables, fince Mr. Geoffroy’'s first attempt, are the cases of DOUBLE
ELECTIVE ATTRACTIONS. Theſe had been but little attended to, or
understood in his time, although they happen in many important ope
rations in chemistry, and the knowledge of them is indispensably ne
cessary on many occasions.
By a double elective attraction, or double exchange, is meant, the
action of two compounds on one another, each of them consisting of
two ingredients, and each of thofe ingredients having an attraction
for one of the ingredients of the other compound. The consequence
of applying two such compounds to one another is, in many cases,
that they are both decompounded, and two new compounds produced
in their stead; each of the ingredients of the one joining itself to that
ingredient of the other compound for which it has an attraction.
As the knowledge of these cases is very useful, I began in the year
1756 or 1757, to represent them by diagrams, and to subjoin theſe to
the table of elective attractions, which I gave to the gentlemen who