324
ELEMENTS OF MATERIA MEDICA.
3. From sulphate of soda. —The principal manufactories are situated inthe northern parts of the kingdom, and are conducted on a most extensivescale. The process adopted varies in some of its details in differentplaces.
The sulphate of soda employed is, in part, obtained from manufacturersof chloride of lime, who procure a considerable quantity in the process,for generating chlorine. But the greater part of it is made expressly byadding sulphuric acid to common salt (chloride of sodium). Thehydrochloric acid which is evolved is allowed to escape into theatmosphere, and is most injurious to animal and vegetable life, as I havebefore stated ( vide p. 150). In the report of a trial at Lancaster,March 21,1838, the Queen v. Airey, in the Times newspaper, is containeda very humorous account of the unpleasant effects of this gas. For thesake of economy, manufacturers of carbonate of soda usually make theirown sulphuric acid.
The sulphate of soda is usually decomposed by mixing it with chalkand some carbonaceous matter (small coal, charcoal, or sawdust), andheating the mixture in a reverberatory furnace. The proportions usedby Leblanc (Dumas, Traite de Chimie, t. 2 me . p. 475) are 1000 parts ofdry sulphate of soda, 1000 of chalk, and 550 of charcoal. In a large ma-nufactory in the neighbourhood of London the proportions of the ingre-,dients employed are nearly 2 parts sulphate, 1 part chalk, and 1 partjsmall coal. The product of this operation lias a dark grey or blackishappearance, and is called English barilla or ball alkali. By lixiviating with!water and evaporating the resulting solution, a blackish crystalline mass is|obtained, which, when roasted,is called soda-ash. This is digested in water,ithe solution evaporated, and the carbonate of soda afterwards crystallized'in iron pans. For other modes of proceeding I must refer to the Traite.'.de Chimie of Dumas, or Brande’s Manual of Chemistry , and Duncan’s!Edinburgh Dispensatory.
The theory of the process is the following: the sulphuric acid ofthe sulphate is deprived of oxygen by the carbonaceous matter (coalor sawdust), while its sulphur is partly burnt and escapes as sul-iphurous acid, and partly combines with the calcium of the chalk to formsulphuret of calcium. The soda unites with carbonic acid produced by'the decomposition-of the chalk, as well as by the combustion of carbon.To purify the impure carbonate of soda of the shops, it is ordered, in thePharmacopoeia , to be dissolved, strained, and re-crystallized.
Properties. —Carbonate of soda usually fonns large crystals, the pri-mitive form of which is, according to Mr. Brooke, the oblique rhombic]prism. They are transparent, and have a cooling alkaline taste. Byexposure to the air they effloresce. When heated they undergo thewatery fusion and give out their water of crystallization: at a red heat, thewhole of the water is expelled, and when the resulting anhydrous earlxHnate has been rubbed to powder it constitutes the soda carbonas exsiccata,Ph. L.: 54 grams of which are equivalent to 144 grains of the crystal-lized salt. Carbonate of soda is insoluble in alcohol. It dissolves intwice its weight of water at 60°, and in less than its own weight at 212° F.The solution reacts as an alkali on vegetable colours.
Characteristics. —As a carbonate it is known by the tests for thisclass of salts already stated (vide p. 301.) From the bicarbonates it isdistinguished by the brick-red precipitate (percarbonate of mercury)