Vide p. 556.

U.S.P. Lithii Salicylas. Salicylate of Lithium. 2LiC7H5O3.H2O; 306.

Characters. - A white powder, deliquescent on exposure to air, odourless, or nearly so, having a sweetish taste and a faintly acid reaction.

Solubility. - Very soluble in water and in alcohol.

Reactions. - When strongly heated the salt chars, emits inflammable vapours, and finally leaves a black residue having an alkaline reaction and imparting a crimson colour to a non-luminous flame. On supersaturating the dilute aqueous solution with hydrochloric acid a bulky white precipitate is obtained, which is soluble in boiling water, from which it crystallises on cooling; also soluble in ether; and producing an intense violet colour with ferric salts.

Uses. - It is used as a remedy in gout and rheumatism, and is intended to unite the properties of salicylic acid and lithium. It is less irritant to the stomach than salicylic acid.

Dose. - 20-40 grs. (1.3-2.6 gm.). Monad Metals. - Group II. Ammonium Salts. Ammonia. NH3; 17.

Ammonium salts are well-defined, like those of potassium and sodium, but the base, instead of being a so-called element, is known to be a compound of nitrogen and hydrogen. They are formed by the direct union of ammonia, NH3, with acids. Thus ammonia and hydrochloric acid unite directly to form ammonium chloride, NH3 + HC1 = NH4C1. In the case of other members of the metallic group this direct union with the components of the acid does not occur, the metal replacing hydrogen, e.g. Zn + 2HC1 = ZnCl2 + H2. This exception to the general rule may be avoided by regarding the compounds of ammonia with acids as not being formed by the direct union of ammonia with the acids, but by the replacement of hydrogen in a basylous radical ammonium, NH4.

In gaseous ammonia the nitrogen may be supposed to be triad with its three affinities saturated by hydrogen, thus,

In the radical ammonium the nitrogen is supposed to be pentad

In the radical ammonium the nitrogen is supposed to be pentad, four of its affinities being saturated by hydrogen, the other being free to unite with an atom of some other element, thus,

In ammonium chloride this free affinity is saturated by chlorine

In ammonium chloride this free affinity is saturated by chlorine

In liquor ammo niae this free affinity is saturated by hydroxyl

In liquor ammo-niae this free affinity is saturated by hydroxyl,

The atoms of hydrogen in ammonia or in ammonium can be replaced by organic radicals

The atoms of hydrogen in ammonia or in ammonium can be replaced by organic radicals, and compound ammonias are formed. When the organic radical which replaces the hydrogen is of a positive nature, the compounds are termed amines, but if it is of a negative nature they are termed amides.

Ammonium, NH4, does not exist in the free state, and whether the double molecule, N2H8 or exists separately is uncertain.

Ammonium, NH4

It has been supposed to form an amalgam with mercury. When mercury, potassium, and sal-ammoniac are mixed, the mercury swells up enormously and forms a pasty amalgam. This may consist of ammonium and mercury, but it soon decomposes into mercury, ammonia, and hydrogen, so that some have supposed it to be nothing more than mercury which has absorbed a certain quantity of gas, as the mercury in this condition yields to pressure in the same way as froth does in other liquids. At all events the salts of ammonium correspond very closely with those of potassium and sodium. In their general reactions they differ, however, in the fact that ammonia is volatile, whereas potassium and sodium are not.

Sources of Ammonium Salts. - Ammonia is formed chiefly by the union of the nitrogen and hydrogen contained in animal or vegetable tissues during the processes of decomposition or destructive distillation. The principal commercial source of ammonium salts is the ammoniacal liquor from gas-works, though some of it is also obtained by the dry distillation of bones in making animal charcoal.

General Reactions of Ammonium Salts. - Like potash and soda, ammonia is not precipitated by most reagents. It is recognised by its volatile alkaline character. It is given off from any of its salts on the addition of caustic potash or soda to them, and is then distinguished by its peculiar smell, and by its volatile alkaline character - turning a piece of red litmus-paper blue and turmeric paper brown, when they are held above the test-glass in which the ammonium salt has been mixed with potash or soda. It also forms white fumes of ammonium chloride when brought near to strong hydrochloric acid.

General Impurities of Ammonium Salts. - As all the salts are obtained from the chloride or sulphate, chlorides or sulphates may be present. Iron may be present, as the chloride is usually sublimed in an iron pot, and, if the heat employed be too great, some ferric chloride sublimes along with the ammonium chloride and gives it a reddish colour. Some lead may also be present from the leaden domes into which the ammonium chloride is sublimed.

General Tests. - Lead and iron are detected by hydrosul-phuric acid, or ammonium sulphide, and iron also by ferrocyanide of potassium. As the gas liquor contains many empyreumatic substances, these may sublime, and they are tested for in carbonate of ammonium (U.S.P.) by solution of permanganate of potassium. The colour of this ought not to alter after standing for five minutes.