788 SODA.—Sodium Sulphate.

able native ash. On the coasts of Spain this ash wasformerly produced so abundantly as to supply, notonly the native demand for soda, but also that ofother countries.

Great Britain produced large quantities of nativesoda under the form of kelp, the manufacture ofwhich was carried on at a very early period on theNorth and West coasts of Ireland , and later on theWestern shores and islands of Scotland . This tradewas especially bi'isk at the commencement of thepresent century, when, to meet the exigencies of thewar, high duties were imposed upon the Spanish barilla. The annual rental of the kelp shores of theisland of North Uist alone amounted at this time to£7000; and at one period the entire quantity ofkelp produced in Scotland and its adjacent islandswas estimated at more than 25,000 tons annually.Still, even this quantity of native soda was not suffi-cient to supply the national needs.

France was especially dependent on Spain forsoda; and when at the time of the French revolu-tion of 1789 the importation of barilla was suspended,the National Convention made, in a special pro-clamation, an appeal to the chemists of France to devise a process by which common salt mightbe rendered available as a source of soda. Tothe appeal of the Convention due response wasmade, and numerous plans were proposed; butof all the processes communicated to the committeeintrusted with the examination and execution of thevarious proposals, the only one deemed likely to besuccessful was that of Leblanc. The first estab-lishment for prosecuting the manufacture on a largescale was erected in 1804 at St. Denis by Leblanc,and was carried out by him and his partners, Dizeand Shee. Leblanc, after having spent all hismeans in perfecting his plan, was neglected by thegovernment of his own country, and though hand-somely rewarded by the British government, at lastdied in an hospital. In Greece and at Koine statueshave since been erected to his memory.

The principle of this process was foreshadowed inEngland by Dr. Brian Higgins as early as 1781. Hispaper shows that he started with common salt, whichhe decomposed into sulphate of soda and hydro-chloric acid by oil of vitroil. The dry sulphate orsalt cake obtained was put into a reverberatoryfurnace, with one-eighth of its weight of coal, andheated till the ingredients were melted, and the sul-phate reduced to the state of sulphide. At thisstage twice its weight of lead was introduced, andwhen the metal was melted, the whole was stirredwith an iron rod until the sulphur united with thelead, and the fused caustic soda floated in a distinctlayer over the surface of the sulphide of lead. Thefurnace was then tapped to allow the alkali to. flowout.

The great expense attending this plan, arisingfrom the high price of lead, rendered it valuelessas a commercial process; but in the same patentHiggins states that iron introduced into the furnace,or other metallic oxides, may he used equally well. Here,therefore, is clearly developed the principle of Le-

blanc’s process. Leblanc, however, had the sagacityto replace the expensive oxide of iron by oxide ofcalcium, and the process practised in Great Britain at the present day is substantially that devised by him.

The first step in the manufacture is the productionof salt cake (sodium sulphate) by the action of oil ofvitriol on common salt.

Sodium Sulphate. — Glauber’s Salt, Salt-cake;Sulfate de Soude, French ; Schwefelsaures Natron,German. Symbol, Na 2 S0 4 . This compound ofsodium was discovered by Glauber in 1658. It isfound copiously in nature as a constituent of min-eral waters, and to a small extent in the mineralform as mirabilite.

Sulphate of soda in the form of mirabilite isfound in Austria, in Bohemia , in the Tyrol, andin several places in Spain and Hungary , as well asin Switzerland ; on lavas, and as an efflorescence onthe Siberian and Caspian steppes. A mixture of thecrystallized salt with gypsum— Glauherite —was dis-covered by Gimbernat in the canton of Argovie inSwitzerland ; the same compound has been foundin crystals and reniform masses, embedded in rock-salt, at Villarubia in Spain , at Vic in Lorraine,Berchtolsgaden in Bavaria , and at Atacama in Chili.

Sodium sulphate is a by-product in the prepara-tion of nitric acid, of sal ammoniac, of carbonate ofmagnesia, in the amalgamation of silver ores ; and itmay also be obtained from many glass slags, if nativeor artificial soda has been used in the preparation ofthe glass.

Pure sodium sulphate is prepared by addition ofsulphuric acid to sodium chloride; hydrochloric acidis set free and sodium sulphate produced. Wheneffervescence has partially subsided heat is applied,and continued till the whole of the hydrochloricacid is expelled.

The dry mass usually contains a little sulphuricacid ; this is removed, after solution in water, by theaddition of a sufficient quantity of carbonate of lime,or the excess of acid may be neutralized by the addi-tion of carbonate of soda. The solution is thenfiltered, evaporated, and set aside to crystallize.

Crystals of sodium sulphate have the composi-tion Na 2 S0 4 + 10H 2 O; their form is the obliquerhombic prism. The anhydrous salt is obtainedby heating the hydrated sulphate to redness or,according to Faraday, by evaporating an aqueoussolution at a temperature considerably below 100°C. The crystals belong to the right prismaticsystem. This latter, which is transparent, whenexposed to the air becomes opaque on the sur-face, owing to absorption of water. The tasteof the crystallized salt is cooling and bitterishsaline. By exposure to the air it effloresces, withthe loss of all its water. The perfectly efflorescedsalt intumesces in very moist air to three times itsbulk, and then becomes a crystalline mass of ordin-ary Glauber’s salt, with a slight excess of water.When the crystallized salt is heated, it fuses,gives off its water, and thereby becomes a whitesolid ; but at a red heat this again becomes liquid:

1 part of the crystals dissolves in 8 parts of water