This liquid is the aqua-regia of the old alchemists. It is produced by mixing nitric and hydrochloric acids: the oxygen contained in the former combines with the hydrogen of the latter, forming water and liberating chlorine, thus:-- NO{5} + HCl = NO{4} + HO + Cl. The presence of free chlorine confers on the mixture the power of dissolving gold and platinum, which neither of the two acids possesses separately. In preparing aqua-regia it is usual to mix one part, by measure, of nitric acid with four of hydrochloric acid, and to dilute with an equal bulk of water. The application of a gentle heat assists the solution of the metal; but if the temperature rises to the boiling point, a violent effervescence and escape of chlorine takes place. Oxygen. Symbol, O. Atomic weight, 8. Oxygen gas may be obtained by heating nitrate of potash to redness, but in this case it is contaminated with a portion of nitrogen. The salt termed chlorate of potash (the composition of which is closely analogous to that of the nitrate, chlorine being substituted for nitrogen) yields abundance of pure oxygen gas on the application of heat, leaving behind chloride of potassium. _Chemical Properties._--Oxygen combines eagerly with many of the chemical elements, forming oxides. This chemical affinity however is not well seen when the elementary body is exposed to the action of _oxygen in the gaseous form_. It is the _nascent_ oxygen which acts most powerfully as an oxidizer. By nascent oxygen is meant oxygen on the point of separation from other elementary atoms with which it was previously associated; it may then be considered to be in the liquid form, and hence it comes more perfectly into contact with the particles of the body to be oxidized. Illustrations of the superior chemical energy of nascent oxygen are numerous, but none perhaps are more striking than the mild and gradual oxidizing influence exerted by atmospheric air, as compared with the violent action of nitric acid and bodies of that class which contain oxygen loosely combined. Oxymel. This syrup of honey and vinegar is prepared as follows:--Take of Honey 1 pound. Acid, acetic, fortiss. (Beaufoy's acid) 11 drachms. Water 13 drachms. Stand the pot containing the honey in boiling water until a scum rises to the surface, which is to be removed two or three times. Then add the acetic acid and water, and skim once more if required. Allow to cool, and it will be fit for use. Potash. Symbol, KO + HO. Atomic weight, 57. Potash is obtained by separating the carbonic acid from carbonate of potash by means of caustic lime. Lime is a more feeble base than potash, but the carbonate of lime, being _insoluble_ in water, is at once formed on adding milk of lime to a solution of carbonate of potash.