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