320
ELEMENTS OF MATERIA MEDICA.
ii. 668) show that the quantity of carbonic acid which it contains islarger than the carbonate, and less than the bicarbonate: hence Mr.Phillips denominated the salt a sesquicarbonate. From the analysis ofMM. Mariano de Rivero and Bossingault (Ann. de Chim. xxix. 110) itappears that the substance termed Urao, and which occurs at the bottomof a lake at Lagunillas, near Merida, in South America , has a similarcomposition.— [For an account of this lake, see Quart. Journ. of Science ,vol. i. p. 188.]
The white powder sold in the shops of this country for making sodaicpowders, and which is denominated carbonate, bicarbonate, or sesquicar-bonate of soda, is a compound of soda and carbonic acid; the quantityof the latter constituent being greater than that of the carbonate, butless than that of the bicarbonate. Hence in the Pharmacopoeia this saltis denominated soda sesquicarbonas, and its composition has been sup-posed to be identical with the trona of Africa and the urao of SouthAmerica . It is distinguished from the ordinary carbonate of soda byits not causing a white precipitate with a cold solution of sulphate ofmagnesia.
The so-called sesquicarbonate of soda of the shops, usually, if notinvariably, contains carbonate and bicarbonate of soda; these may bedetected as follows:—Wash with a small quantity of distilled water, andfilter: the solution contains carbonate of soda (known by its throwingdown a brick-dust red precipitate on the addition of bichloride of mer-cury)—while there remains on the filter bicarbonate of soda (recognizedby its causing a white precipitate, or a slight milkiness or opalescencewith bichloride of mercury.)* The relative proportions of carbonate andbicarbonate of soda are not constant—a fact which will explain the fol-lowing remark made by Mr. Phillips ( Transl. of the Pharm. 3 rd . ed.1838):—“ I am informed by Mr. Everitt that bicarbonate of soda is nownot unfrequcntly to be met with; and very commonly, instead of meresesquicarbonate, a mixture of this salt and a large proportion of bicar-bonate may be obtained from those who manufacture on a large scale.”(For further information respecting the soda sesquicarbonas, Pli. L., secSob.-e Bicarronas.)
The composition of native crystallized sesquicarbonate of soda is asfollows:—
Eq.
Eq.Wt.
Per Cent.
Trona.
Urao.
Soda .
. . 1 .
. . 32 . .
. . 38-55 .
. . . 37-0 .
. . . 38-62
Carbonic Acid
• • H •
. . 33 . .
. . 39-76 .
. . . 38-0 .
. . . 40-13
Water.
. . 2 .
. . 18 . .
. . 21-69 .
. . . 22-5 .
. . . 21-24
Sulphate Soda
• * — •
. - - - .
. . . 2-5 .
... —
Native Sesquicarbonate Soda 1 . .
. . 83 . .
. . 100-00 .
. . 100-0 .
. . . 99-99
* I have been recently informed by Mr. Scanlan that the salt called Ammonia Ses-quicarbonas (vide p. 173) is resolved, by washing with water, into two salts, the car-bonate of ammonia, which dissolves, and a mass of crystals of bicarbonate of ammoniahaving the form of the lump of sesquicarbonate employed. Mr. Scanlan has alsoobligingly furnished me with illustrative specimens of the above results. The crystalsof what he has sent me as bicarbonate of ammonia have, however, a slight odour ofammonia; but be tells me, that when first prepared they were odourless. Dr. Dalton(Mem. of the Lit. Sf Phil. Soc. of Manchester, 2 nd Ser. vol. iii. p. 18) seems to havebeen aware that the salt now called sesquicarbonate of ammonia was a mixture oftwo salts.