_Sulphates_ are estimated by precipitation as barium sulphate and calculated to Na_{2}SO_{4}. _Sodium._--This may be determined by converting the salt into sodium sulphate by the action of concentrated sulphuric acid, igniting to drive off hydrochloric and sulphuric acids, and fusing the mass until constant in weight, weighing finally as Na_{2}SO_{4}. POTASSIUM CHLORIDE. This should be examined, in the same way as sodium chloride, for chloride, insoluble matter, 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.