ter, moisture, and sulphate. The potassium may be
determined as potassium platino-chloride, as described under carbonate
of potash.

SILICATES OF SODA AND POTASH.

The most important determinations for these are total alkali and silica.

_Total alkali_ is estimated by dissolving 2 grammes in distilled water,
and titrating when cold, with N/1 acid, using methyl orange as
indicator.

_Silica_ may be determined by dissolving 1 gramme in distilled water,
rendering the solution acid with HCl, and evaporating to complete
dryness on the water-bath, after which the residue is moistened with HCl
and again evaporated, this operation being repeated a third time. The
residue is then heated to about 150 deg. C., extracted with hot dilute HCl,
filtered, thoroughly washed, dried, ignited in a tared platinum
crucible, and weighed as SiO_{2}.

ESSENTIAL OILS.

As already stated, these are very liable to adulteration, and an
examination of all kinds of oil is desirable, while in the case of the
more expensive varieties it should never be omitted.

_Specific Gravity._--As with fats and oils, this is usually taken at 15 deg.
C., and compared with water at the same temperature. In the case of otto
of rose and guaiac wood oil, however, which are solid at this
temperature, it is generally observed at 30 deg. C. compared with water at
15 deg. C.

The specific gravity is preferably taken in a bottle or U-tube, but if
sufficient of the oil is available and a high degree of accuracy is not
necessary, it may be taken either with a Westphal balance, or by means
of a hydrometer.

_Optical Rotation._--For this purpose a special instrument, known as a
polarimeter, is required, details of the construction and use of which
would be out of place here. Suffice it to mention that temperature plays
an important part in the determination of the optical activity of
certain essential oils, notably in the case of lemon and orange oils.
For these Gildemeister and Hoffmann give the following corrections:--

Lemon oil, below 20 deg. C. subtract 9' for each degree below, above 20 deg. C.
add 8' for each degree above.

Orange oil, below 20 deg. C. subtract 14' for each degree below, above 20 deg.
C. add 13' for each degree above.

_Refractive Index._--This figure is occasionally useful, and is best
determined with an Abbe refractometer, at 20 deg. C.

_Solubility in Alcohol._--This is found by running alcohol of the
requisite strength from a bur