_Properties of Ether._--It is neither acid nor alkaline to test-paper. Specific gravity, at 60°, about ·720. Boils at 98° Fahrenheit. The vapor is exceedingly dense, and may be seen passing off from the liquid and falling to the ground: hence the danger of pouring ether from one bottle to another if a flame be near at hand. Ether does not mix with water in all proportions; if the two are shaken together, after a short time the former rises and floats upon the surface. In this way a mixture of ether and alcohol may be purified to some extent, as in the common process of _washing_ ether. The water employed however always retains a certain portion of ether (about a tenth part of its bulk), and acquires a strong ethereal odor; washed ether also contains water in small quantity. Bromine and iodine are both soluble in ether, and gradually react upon and decompose it. The strong alkalies, such as potash and soda, also decompose ether slightly after a time, but not immediately. Exposed to air and light, ether is oxidized and acquires a peculiar odor. Ether dissolves fatty and resinous substances readily, but inorganic salts are mostly insoluble in this fluid. Hence it is that iodide of potassium and other substances dissolved in alcohol are precipitated to a certain extent by the addition of ether. Fluoride of Potassium. Symbol, KF. Atomic weight, 59. _Preparation._--Fluoride of potassium is formed by saturating hydrofluoric acid with potash, and evaporating to dryness in a platinum vessel. _Hydrofluoric acid_ contains fluorine combined with hydrogen; it is a powerfully acid and corrosive liquid, formed by decomposing flour spar, which is a _fluoride of calcium_, with strong sulphuric acid; the action which takes place being precisely analogous to that involved in the preparation of hydrochloric acid. _Properties._--A deliquescent salt, occurring in small and imperfect crystals. Very soluble in water: the solution acting upon glass in the same manner as hydrofluoric acid. Formic Acid. Symbol, C{2}HO{3}. Atomic weight, 37. This substance was originally discovered in the _red ant_ (_Formica rufa_), but it is prepared on a large scale by distilling _starch_ with binoxide of manganese and sulphuric acid. _Properties._--The strength of commercial formic acid is uncertain, but it is always more or less dilute. The strongest acid, as obtained by distilling formiate of soda with sulphuric acid, is a fuming liquid with a pungent odor, and containing only one atom of water: it inflames the skin in the same manner as the sting of the ant. Formic acid reduces the oxides of gold, silver, and mercury, to the metallic state, and is itself oxidized into carbonic acid. The alkaline formiates also possess the same properties. Gelatine. Symbol, C{13}H{10}O{5}N{2}. Atomic weight, 156.