CARBONATE OF AMMONIA.—On tho large scab., this salt is obtained by heating sal-ammoniac, or the sulphate of ammonia, with chalk (carbonate of lime, or carbonate of potash or soda) ; the carbonate sublimca and condenses on the aides of the receiver. Tho receivers are constructed of lead, iron or earthenware; the latter is used in the manufacture of the salt used in medicine and other purposes requiring a pure salt of ammonia ; the inside of these receivers are sometimes lined with a coating of clay, or prepared canvas and clay, which prevents the metallic impregnation, and at the same time allows the sublimate to be more easily detached. The receivers are made in sections bolted together and luted at the junctions. It is easier to obtain the purified salt by employing a well prepared chloride or sulphate, than to attempt to resublime an impure carbonate, as the impurities are carried into the condensers mechanically.
The carbonates of ammonia of commerce have a variablo composition, and consist principally of a aesqui-oarbonate mixed with other carbonates. Carbonate of ammonia is met with in commerce in irregular pieces about three-fourths of an inch thick, with a slightly crystalline fracture when broken, which crumble and pass into an opaque mass on exposure to the air. The carelessly prepared salt may contain more or less of the impurities contained in the commercial muriatio or sulphuric acids, a matter of great importance, espeoially when used in pharmacy and chemical analyses.
The amount of carbonic acid in a sample of carbonate may be obtained from the loss when a weighed quantity is decomposed in a carbonic acid apparatus. The salt dissolved in hydrochloric acid in excess and evaporated to dryness will give the weight of ammonia as chloride. The difference, if any, will be due to water. It should dissolve completely in water, and the solntion should undergo no change on the addition of a little sulphide of ammonium. A portion of the salt dissolved in nitrio acid in excess should produce no change in a solution of nitrate of silver.
Several methods have been introduced for tho production of this salt on a commercial scale. As far back as 1825, Mr. Holmes, of Liverpool, manufactured this salt from stale or fermented urine it is curious to note that in 1867 it was proposed to manufacture ammonia from the same article, at Bayeres. In 1844, Dr. Turner obtained a patent for distilling guano to obtain salts of ammonia, and
in 1849, Mr. Hills secured a patent for distilling a mixture of charcoal or coke and guano for the same purpose, and in the same year the same inventor obtained carbonate of ammonia by the action of heat on peat.
At the present time the crude carbonate is obtained from a mixture of the chloride or sulphate of ammonia and common chalk, which is heated in retorts and sublimed, the chloride or sulphate being first obtained from "gas liquor" as follows :—The free ammonia is first driven off and received in condensers with muriatie or sulphuric acid, the combined ammonia which remains in the retorts is drawn off with the liquids still remaining, and sufficient acid is added to convert the ammonia compounds into chloride, or sulphate. This solution, after allowing the impurities to subside, is decanted; taking care not to allow the floating film of oil and tarry matter to run off, and evaporated in large hemispherical iron pans set in brickwork, and which are generally heated by the waste heat from the furuaces. The crystals obtained are more or less black from tarry matter ; when drained and slightly washed they are redissolved, a quantity of matter separates, which had been mechanically mixed with the crystals ; the solution should be syphoned off, evaporated and set to crystallize in shallow iron or lead pans. This yields crystals of a dirty white colour, but are so far freed from volatile tarry matter, that they are now ready for conversion into carbonate.
For this purpose cast iron retorts, the shape of an elongated muffle, are used. They are shown in Fig. 196, in which A is the ashpit, and B the fireplace ; C C C are the retorts, S S S S the subliming-pots, D D the condensing chambers, T T T T pipes leading from the retorts, and 0 an outlet for the steam and vapours. is an earthenware subliming-pot ; 0' a retort, with a rod R for clearing the outlet to the condensing chambers ; M is the top of the furnace, with spaces H for the subliming-pots. In Fig. 197, S' is a leaden subliming-pot in two halves, with an iron ring, and S" another, made of bent sheet lead, with a top.