[Footnote 1: Read before the Birmingham Philosophical Society,
January 22, 1885.]
By THOMAS TURNER, Assoc. R.S.M., F.C.S., Demonstrator of Chemistry,
Mason College.
There are a number of interesting facts, some of which are known to
most persons, and many of them have been long recognized, of which,
however, it must be owned that the explanation is somewhat obscure,
and the connections existing between them have been but recently
pointed out. As an example of this, it is well known that salt water
freezes at a lower temperature than fresh water, and hence sea-water
may be quite liquid while rivers and ponds are covered with ice.
Again, it is noticed that mixtures of salts often have a fusing-point
lower than that of either of the constituent salts, and of this fact
we often take advantage in fluxing operations. Further, it is well
known that certain alloys can be prepared, the melting-points of which
are lower than the melting-point of either of the constituent metals
alone. Thus, while potassium melts at 62.5° C., and sodium at about
98°, an alloy of these metals is fluid at ordinary temperatures, and
fusible metal melts below the temperature of boiling water, or more
than 110° lower than the melting-point of tin, the most fusible of the
three metals which enter into the composition of this alloy. But
though these and many similar facts have been long known, it is but
recently, owing largely to the labors of Dr. Guthrie, that fresh
truths have been brought to light, and a connection shown to exist
throughout the whole which was previously unseen, though we have still
to acknowledge that at present there is much at the root of the matter
which is but imperfectly understood. Still Dr. Guthrie proves a
relationship to exist between the several facts we have previously
mentioned, and also between a number of other phenomena which at first
sight appear to be equally isolated and unexpected, and we are asked
to regard them all as examples of what he has called "eutexia."
We may define a eutectic substance as a body composed of two or more
constituents, which constituents are in such proportion to one another
as to give to the resultant compound body a minimum temperature of
liquefaction--that is, a lower temperature of liquefaction than that
given by any other proportion.[2] It will be seen at once by this
definition that the temperature of liquefaction of a eutectic
substance is lower than the temperature of liquefaction of either or
any of the constituents of the mixture. And, further, it is plain that
those substances only can be eutectic which we can obtain both as
liquid and solid, and hence the property of eutexia is closely
connected with solution.
[Footnote 2: Guthrie, _Phil. Mag._ [5], xvii., p. 462.]
Following in the natural divisions adopted by Dr. Guthrie, we may
consider eutexia in three aspects: