white oxyd of arsenic in solution, owing to
its not being sufficiently oxygenated; this is prevented by continuing
to add nitrous acid, as in the former process, till no more nitrous gas
is produced. From all these observations I would give the following
definition of arseniac acid. It is a white concrete metallic acid,
formed by the combination of arsenic with oxygen, fixed in a red heat,
soluble in water, and capable of combining with many of the salifiable
bases.

SECT. XXIV.--_Observations upon Molybdic Acid, and its Combinations with
Acidifiable Bases[43]._

Molybdena is a particular metallic body, capable of being oxygenated, so
far as to become a true concrete acid[44]. For this purpose, one part
ore of molybdena, which is a natural sulphuret of that metal, is put
into a retort, with five or six parts nitric acid, diluted with a
quarter of its weight of water, and heat is applied to the retort; the
oxygen of the nitric acid acts both upon the molybdena and the sulphur,
converting the one into molybdic, and the other into sulphuric acid;
pour on fresh quantities of nitric acid so long as any red fumes of
nitrous gas escape; the molydbena is then oxygenated as far as is
possible, and is found at the bottom of the retort in a pulverulent
form, resembling chalk. It must be washed in warm water, to separate any
adhering particles of sulphuric acid; and, as it is hardly soluble, we
lose very little of it in this operation. All its combinations with
salifiable bases were unknown to the ancient chemists.

TABLE _of the Combinations of Tungstic Acid with the Salifiable Bases._

_Bases._ _Neutral Salts._

Lime Tungstat of lime.
Barytes barytes.
Magnesia magnesia.
Potash potash.
Soda soda.
Ammoniac ammoniac.
Argill argill.
Oxyd of antimony(A), &c. antimony(B), &c.

[Note A: The combinations with metallic oxyds were set down by Mr
Lavoisier in alphabetical order; their order of affinity being unknown,
I have omitted them, as serving no purpose.--E.]

[Note B: All these salts were unknown to the ancient chemists.--A.]

SECT. XXV.--_Observations upon Tungstic Acid, and its Combinations._

Tungstein is a particular metal, the ore of which has frequently been
confounded with that of tin. Th