magnesia in very fine powder which had been previously deprived of its
air by fire; and observing that the magnesia had a tendency to concrete
into a solid mass, I shook the vial very frequently. After some days the
powder was increased to more than double its former bulk; and when the
vial was opened, the alkaline spirit emitted a most intolerably pungent
smell. It likewise floated upon water, but was not perfectly caustic;
for it still yielded some air when mixed with acids, and also rendered
lime-water turbid: neither of which would probably have happened if I
had used a greater quantity of magnesia, or had allowed the mixture to
remain a longer time in the vial. I now washed out the whole of the
mixture into a bowl, and dryed the magnesia until it lost all smell of
the alkali. It weighed a dram and fifty eight grains, effervesced
violently with acids, and therefore contained a large quantity of air,
which had been drawn from the alkali by a stronger attraction.
Having formerly shewn, that magnesia saturated with air separates an
acid from a calcarious earth, which it is not able to do after being
deprived of its air by fire; I now suspected that the air was the cause
of this separation, because I found that it was joined to the calcarious
earth at the same time that the acid was joined to the earth of
magnesia; and imagined that a pure calcarious earth might possibly have
a stronger attraction for acids than a earth of magnesia.
I therefore dissolved two drams of magnesia in the marine acid, and
thus obtained a compound of an acid and of the pure earth of this
substance; for the air which was at first attached to it, was expelled
during the dissolution. I then added thirty grains of strong quick-lime
in exceeding fine powder, shook the mixture well, and filtrated it. The
powder remaining in the paper, after being well washed, was found to be
a magnesia, which, as I expected, was destitute of air; for it was
dissolved by the vitriolic acid without effervescence. And the filtrated
liquor contained the lime united to the acid; for upon dropping spirit
of vitriol into it, a white powder was immediately formed.
We must therefore acknowledge a stronger attraction between the
calcarious earths and acids than between these and magnesia: but how
does it then happen, that, if magnesia saturated with air be mixed with
a compound of acid and calcarious earth, these two last, which attract
one another the most strongly, do not remain united; but the acid is
joined to the magnesia, and the calcarious earth to the air which it
attracts much more weakly than it does the acid? Is it because the sum
of the forces which tend to join the magnesia to the acid and the
calcarious earth to the air, is greater than the sum of the forces which
tend to join the calcarious earth to the acid, and the magnesia to the
air: and because there is a repulsion between the acid and air, and
between the two earths; or they are somehow kept asunder in such a
manner as hinders any three of them from being united together?
The first part of this supposition is favoured by our experiments, which
seem to shew a greater difference between the forces wherewith the
calcarious earth and magnesia attract fixed air, than between those
which dispose them to unite with the acid. The repulsions however hinted
in the second are perhaps more doubtful, tho' they are suggested in many
other instances of decomposition; but the bounds of my present purpose
will not allow me to enter upon this subject, which is one of the most
extensive in chemistry.
We meet also with a difficulty with respect to the volatile alkali
similar to the above. Thus a calcarious earth that is pure or free of
air has a much stronger attraction for acids than a pure volatile
alkali, as is evident when we mix quick-lime with salt ammoniac; for the
alkali is then immediately detached from the acid: and agreeably to this
I found, upon trial, that a pure or caustic volatile alkali does not
separate a calcarious earth from an acid. Yet, if we mix a mild volatile
alkali, which is a compound of alkali and air, with a compound of acid
and calcarious earth, these two last, which attract one another most
strongly, do not remain united; but the acid is joined to the alkali and
the earth to the air, as happens in the precipitation of a calcarious
earth from an acid, by means of the common or mild volatile alkali.
I remember likewise a parallel instance with regard to quick-silver.
This metal has an attraction for the vitriolic acid, and when joined to
it appears under the form of turbith mineral: but this attraction is
weaker than that of the fixed alkali for the same acid; for if we mix a
dissolved salt of tartar with turbith mineral, the turbith is converted
into a brown powder, and the alkali into vitriolated tartar; which
change happens the sooner, if the pure or caustic alkali is used. Yet,
if to a compound of quick-silver and the nitrous acid, we add a compound
of the fixed alkali and the vitriolic acid, or a vitriolated tartar, and
digest the mixture with a strong heat, the vitriolic acid does not
remain with the alkali, but is joined to the quick-silver which it
attracts more weakly, composing with it a turbith mineral; while the
alkali is joined to the nitrous acid which it likeways attracts more
weakly than it does the vitriolic, and is converted into salt-petre.
From some of the above experiments, it appears, that a few alterations
may be made in the column of acids in Mr. _Geoffroy's_ table of elective
attractions, and that a new column may be added to that table, according
to the following scheme, where the alkaline substances are all
considered as in their pure state and free of fixed air.
Acids. Fixed air.
------------------------------ -----------------
Fixed alkali, Calcarious earth.
Calcarious earth, Fixed alkali.
Volatile alkali and magnesia. Magnesia.
Volatile alkali.
---------------------- ---------------
At the foot of the first column several of the metals might follow, and
after these the earth of alum; but as I don't know what number of the
metals should precede that earth, I have left it to be determined by
further experience.
The volatile alkali and magnesia are placed in the same line of this
column; because their force of attraction seems pretty equal. When we
commit a mixture of magnesia and salt ammoniac to distillation, the
alkali arises and leaves the acid with the magnesia; because this earth,
by attracting the acid, represses its volatility, and it seems also to
diminish the cohesion of the acid and alkali, and to render them
separable by a gentle heat. If the magnesia be saturated with air, this
likewise, on account of its volatile nature and attraction for the
alkali, is driven up along with it, and makes it appear under a mild
form, and in the same manner do the alkali and air arise from a mixture
of salt ammoniac and of a crude calcarious earth.
Footnotes:
[1] June 5. 1755.
[2] Hoff. Op. T. iv. p. 479.
[3] Hoff. Op. T. iv. p. 500.
[4] Mr. _Margraaf_ has lately demonstrated, by a set of curious and
accurate experiments, that this powder is of the nature, and possesses
the properties, of the gypseous or selenitic substances. That such
substances can be resolved into vitriolic acid and calcarious earth, and
can be again composed by joining these two ingredients together. Mem. de
l'Acad. de Berlin. an. 1750, p. 144.
[5] Hoff. Op. T. iv. p. 480 & 500.
[6] Mem. de l'Acad. de Berlin. an. 1748, p. 57.
[7] Hoff. Op. T. iv. p. 480.
[8] This evaporation was performed in a silver dish, on account of the
acrimony of the salt; which is so very great, that, having once
evaporated a part of the same ley in a bowl of English earthen or stone
ware, and melted the caustic with a gentle heat, it corroded and
dissolved a part of the bowl, and left the inside of it pitted with
small holes.
[9] Mem. de l'Acad. de Berlin. an. 1746, p. 87.
[10] Boerh. Operat. Chem. process. LXXVI.
Transcribers Notes:
1. Author's spelling has been retained.
2. Minor punctuation issues have been corrected without note.