CHAMBERS'S INFORMATION FOR THE PEOPLE. 



of nitrogen, and only atmospheric nitrogen, with 

 any excess of oxygen which may be present, 

 escapes at the top of the tower. As nitric oxide 

 is not soluble in sulphuric acid, it would also 

 escape, and thus be lost, if it were not all con- 

 verted into peroxide of nitrogen before leaving the 

 last chamber. The solution of peroxide of nitro- 

 gen is conducted back to the first chamber, where 

 the actions (3) (2) are again commenced. It will 

 be seen that, theoretically, a given quantity of 

 nitric acid should be able to convert an unlimited 

 quantity of sulphurous acid, water, and oxygen, 

 into sulphuric acid. In practice, however, this is 

 not the case, and it is necessary to supply new 

 nitric acid from time to time to compensate for 

 loss. This loss occurs (not to speak of accidental 

 leakage) from two causes : i. Part of the nitric 

 oxide, even in the presence of excess of oxygen, is 

 reduced to nitrous oxide (N 2 O), which, not being 

 directly oxidisable, takes no further part in the 

 action, and escapes with the nitrogen from the 

 top of the tower. 2. The sulphuric acid on the 

 floor of the chamber, although not strong enough 

 to dissolve peroxide of nitrogen, dissolves some 

 nitric acid, which is removed with it from the 

 chamber. The admission of steam to the chambers 

 must be so regulated, that the sulphuric acid on 

 the floor has the specific gravity I -5, which corre- 

 sponds to 60 per cent of hydrated sulphuric 

 acid (H 2 O,SO3), and 40 per cent, water. If the 

 acid is stronger than this, the oxidation does not 

 take place regularly ; and if it is weaker, it dis- 

 solves sulphurous acid. The 'chamber acid' is 

 concentrated by heating it in glass or platinum 

 vessels, until it has a specific gravity 1-83, corre- 

 sponding to about 92 per cent, of hydrated sul- 

 phuric acid. The concentration can be carried 

 on up to specific gravity 1-75 in lead vessels ; acid 

 stronger than this acts rapidly on lead at high 

 temperatures. 



Sulphuric acid thus prepared contains various 

 impurities : i. Sulphate of lead, derived from a 

 slight action on the lead of the chambers, or on 

 the lead-pan used for concentration ; sulphate of 

 lead, being insoluble in dilute sulphuric acid, is 

 precipitated as a white powder -when sulphuric 

 acid containing it is mixed with water. 2. Nitric 

 acid. 3. Arsenic, derived from the iron pyrites, 

 which always contains traces of arsenic. Sulphuric 

 acid prepared from sulphur does not contain 

 arsenic. 



MANUFACTURE OF SODA. 



Carbonate of soda occurs naturally in Hungary, 

 North Africa, and Mexico. It is found either 

 as a crystalline crust, left by the evaporation of 

 the water of lakes containing the salt in solution, 

 or mixed with earth. In the latter case, the earth 

 is exhausted with water, and the solution evapor- 

 ated. The North African soda, from Egypt and 

 Fezzan, has been long known under the names 

 'natron' and 'trona.' It is the substance men- 

 tioned in the Bible (Proverbs xxv. 20 ; Jeremiah 

 ii. 22) as nitre : the reader will see, on referring 

 to the passages, that the meaning becomes 

 at once clear if we read 'washing-soda,' instead 

 of 'nitre.' 



Another long-known source of soda is the ash 

 of certain plants. Just as most land-plants con- 

 tain organic salts of potash, which, when the 



338 



plants are burnt, are converted into carbonate of 

 potash, which can be extracted from the ash so 

 sea-weeds and some plants growing on the sea- 

 shore contain organic salts of soda, and in pre- 

 cisely the same way their ashes yield carbonate of 

 soda. ' Barilla ' is the crude carbonate of soda 

 obtained in Spain from the ash of the sea-side 

 plant, Salsola soda; and in France, ' salicor ' and 

 'blanquette' are similarly obtained from the ashes 

 of Salicornia annua and other shore-plants. 

 ' Kelp,' the ash of various sea-weeds gathered and 

 burnt on the northern and western shores of 

 Scotland and Ireland, also contains carbonate of 

 soda, but is chiefly prepared on account of the 

 iodides contained in it (see CHEMISTRY). 



But by far the greater part of the soda of com- 

 merce is prepared from common salt (chloride of 

 sodium), by what is called, from its inventor, 

 ' Leblanc's process.' 



The salt is first heated with sulphuric acid, and 

 thus converted into sulphate of soda ; the hydro- 

 chloric acid evolved being condensed in coke 

 towers, down which a stream of water flows : 



NaCl + H 2 SO 4 = HC1 + NaHSO 4 and 

 Nad + NaHSO 4 = HC1 + Na,jSO 4 . 



(See CHEMISTRY.) The sulphate of soda, known as 

 ' salt-cake,' is then mixed with coal and limestone 

 in the proportion of about 2 parts salt-cake, 2 lime- 

 stone, and rather more than I coal. The mixture 

 is heated in a reverberatory furnace, and thoroughly 

 stirred with an iron instrument. After about an 

 hour's heating, it is removed, and allowed to cool. 

 In this state it is called ' black ball.' The precise 

 course of the chemical change is not yet certainly 

 ascertained, but the final result is, that the ' black 

 ball' consists chiefly of carbonate of soda and 

 a compound of lime with sulphide of calcium 

 (3CaS,CaO), along with some caustic soda and 

 charcoal. By treatment with water, the carbonate 

 of soda and caustic soda are dissolved, while the 

 oxysulphide of calcium and the charcoal are left 

 behind. The clear solution is then evaporated, 

 and the dried residue is known in commerce as 

 ' soda-ash.' 



' Soda crystals,' or ' washing-soda,' is prepared 

 from soda-ash by dissolving it in hot water, 

 and allowing the solution to crystallise, when 

 nearly pure carbonate of soda crystallises out 

 with 10 molecules of water of crystallisation 

 (Na 2 O,CO 2 ,ioH 2 O or Na 2 CO 3 ,ioH 2 O), almost all 

 the impurities remaining in the mother-liquor 

 (see CHEMISTRY). 'Bicarbonate of soda,' or 

 ' baking-soda,' is prepared by treating soda crystals 

 with carbonic acid gas. 



The residue left, on dissolving out the carbonate 

 of soda from the ' black ball,' consisting chiefly of 

 oxysulphide of calcium (the compound of lime and 

 sulphide of calcium), is called ' soda-waste,' and 

 has long been a source of great annoyance to soda 

 manufacturers and their neighbours. It accu- 

 mulates in vast mounds round the works, and 

 gives off sulphuretted hydrogen whenever acid 

 liquids come in contact with it. 



There are now various methods in use by which 

 the sulphur is recovered from the waste, so that 

 it can be used again in the manufacture of sul- 

 phuric acid. 



These methods are in principle the same, and 

 consist in the oxidation of the waste by means of 

 atmospheric oxygen, and in the precipitation of 



