_b. Sand._--As this substance possesses a much greater specific gravity than the usual constituents of flour, it is only necessary, in order to accomplish its separation, to repeatedly stir the flour with water, and remove the deposit at first formed, which, if consisting of sand, will be insoluble in acids, and will grate, when placed between the teeth. _c. Carbonates of lime and magnesia; vegetable ashes._--Carbonic acid is always evolved, upon treating flour with hydrochloric acid. If the base present be calcium, upon adding oxalate of ammonia to the filtered solution--which has previously been neutralized with ammonia--a white precipitate, possessing the properties mentioned above, will be formed; in case the base is magnesia, the addition of oxalate of ammonia will fail to cause a precipitate, but upon adding solution of phosphate of ammonia to the fluid a granular precipitate of phosphate of ammonia and magnesia is produced; if, finally, the flour contains vegetable ashes--_i. e._ carbonates of the alkalies--bichloride of platinum will produce in the acid solution a yellow precipitate: the addition of vegetable ashes, moreover, would render the ash of the flour deliquescent and very strongly alkaline. _d. Lime._--In presence of lime, carbonic acid produces a white precipitate, when conducted into the filtered aqueous extract of the flour. _e. Plaster._--The flour is boiled with water acidulated with hydrochloric acid, the fluid filtered, and lime detected in the filtrate by means of ammonia and oxalate of ammonia. The presence of sulphuric acid is indicated by the formation of a precipitate insoluble in acids, upon addition of solution of chloride of barium. Upon calcining the flour without access of air, sulphate of lime is converted into the corresponding sulphide: the residue of the calcination, when treated with hydrochloric acid, evolves sulphuretted hydrogen, and the lime present in the filtered acid solution is likewise precipitated by the addition of ammonia and oxalate of ammonia. _f. Alum._--A portion of the flour to be examined is treated with water, the fluid filtered, and the filtrate divided in two portions: in one, sulphuric acid is detected by means of chloride of barium; in the other, alumina by adding a solution of potassa, which gives with its salts a white gelatinous precipitate, soluble in an excess of the reagent.[R]