CHEMISTRY APPLIED TO THE ARTS. 



A LMOST every branch of manufacture involves 

 /X to a greater or less extent chemical principles, 

 so that a complete treatise on chemical tech- 

 nology should contain an account of nearly every 

 manufacturing process, and a discussion of the 

 chemical actions taking place in each. It is ob- 

 vious that nothing of this kind can be attempted 

 here. We shaU confine ourselves to a statement 

 of the chemical principles involved in a few of the 

 more important chemical manufactures, excluding, 

 of course, those to which separate papers are 

 devoted in this series, or which have been suffi- 

 ciently explained in the article CHEMISTRY. 



It is scarcely possible to adopt anything like a 

 systematic arrangement of this subject, but we 

 shall endeavour to keep, as far as possible, allied 

 manufactures together, and to proceed from the 

 less to the more complex. The reader is expected 

 to possess an elementary knowledge of chemistry, 

 such as might be acquired by the study of the 

 paper on CHEMISTRY. 



We shall first consider the preparation of sul- 

 phuric acid, as that is the starting-point of a large 

 number of chemical manufactures. 



Anhydrous sulphuric acid (SO 3 ) can be prepared 

 by heating certain sulphates. Thus, if bisulphate 

 of soda (H 2 O,SO 3 ,Na 2 O,SO 3 or HNaSO^ is 

 heated, it loses water, and is converted into 

 what is called anhydrous bisulphate of soda 

 (Na 2 O,SO 3 ,SO 3 ). When this substance is very 

 strongly heated, it gives off anhydrous sulphuric 

 acid, and normal sulphate of soda is left. The 

 ferric sulphates are decomposed by heat into 

 ferric oxide and anhydrous sulphuric acid. Thus, 

 Fe 2 O 3 ,3SO 3 = Fe 2 O 3 + 3SO 3 ; and a similar 

 decomposition takes place with basic ferric sul- 

 phates. Thus, Fe 2 O 3 ,2SO 3 = Fe 2 O 3 + 2SOg. It 

 is upon this latter decomposition that the prepar- 

 ation of 'Nordhausen' or ' Saxon' sulphuric acid 

 depends. 



Green vitriol or ferrous sulphate has the com- 

 position FeSO 4 ,7H 2 O or FeO,SO 3 ,7H 2 O ; when 

 exposed to heat and air, it absorbs oxygen, and 

 is converted into basic ferric sulphate; thus, 

 2(FeO,SO 3 ) + O = Fe 2 O 3 ,2SO 3 , the water of 

 crystallisation being given off. If the oxidation 

 and drying were complete, on distilling the pro- 

 duct nothing would be given off but anhydrous 

 sulphuric acid. But in practice this is not so ; 

 some water is always left, so that some hydrated 

 sulphuric acid (H 2 O,SO 3 or H-jSO^ is formed, 

 and some ferrous sulphate (FeOjSOj) is always 

 left unoxidised, which undergoes the following 

 decomposition : 2(FeO,SO 3 ) = Fe 2 O 3 + SO 3 + 

 SO 2 , half of the SO 3 losing oxygen, so as to con- 

 vert 2FeO into Fe 2 O 3 . The result, therefore, is, 

 that ferric oxide (Fe 8 Os) is left behind, and sul- 

 phurous acid (SOj), anhydrous sulphuric acid 

 (SOa), and hydrated sulphuric acid (H 2 O,SO 3 

 or H 2 SO 4 ) pass over. The latter unite to 

 form 'fuming' or 'Nordhausen' sulphuric acid 

 (H 2 O,2SO 3 or H 2 S 2 O r ). The distillation is per- 

 formed in earthenware retorts, arranged in rows in 

 22 



a furnace, and earthenware jars are used as re- 

 ceivers. This mode of preparing sulphuric acid 

 from 'green vitriol ' gave rise to the name ' oil of 

 vitriol,' now generally given to hydrated sulphuric 

 acid. 



But by far the largest part of the sulphuric acid 

 of commerce is prepared from sulphurous acid. 



The sulphurous acid is made either by burning 

 sulphur (S + O 2 = SO 2 ), or, more commonly, by 

 burning iron pyrites (FeS 2 ). When this mineral is 

 heated in presence of air, it burns, giving out a 

 great deal of heat, and forms ferric oxide and sul- 

 phurous acid; thus, 2FeS 2 + ;O = Fe 2 O 3 + 

 2SO 2 . The sulphurous acid gas, prepared in 

 either way, is led by a wide pipe into a series of 

 large chambers lined with lead, into which nitric 

 acid, atmospheric air, and steam, are also con- 

 ducted. The action which takes place is repre- 

 sented by the following equations : 



(i.) 3SO, + H,0,N 2 S + 2H 2 = 3(H 2 0,S03) + 2NO. 



The sulphuric acid (H 2 O,SO 3 ) falls down as a fine 

 rain to the bottom of the chamber, while the 

 nitric oxide (NO) (see CHEMISTRY) immediately 

 unites with oxygen from the air to form peroxide 

 of nitrogen, thus : 



(2.) NO + O = NO a . 



The peroxide of nitrogen then acts on sulphuric 

 acid and water, thus : 



(3.) N0 a + H 2 O + SO 2 = NO + H 2 O,S0 3 . 



The sulphuric acid thus formed falls to the floor 

 of the chamber, and the nitric acid (NO) unites 

 with oxygen from the air, as in (2). If atmos- 

 pheric air is admitted in sufficient quantity, the 

 actions (2) and (3) go on continuously, a limited 

 quantity of oxide of nitrogen being able to convert 

 an unlimited quantity of sulphurous acid and 

 water into sulphuric acid ; the nitrogen oscillating 

 between two states of oxidation, NO and NO.,, 

 the latter giving oxygen to the sulphurous acid, 

 and the former taking oxygen from the air. The 

 gas in the chambers, therefore, becomes poorer 

 in sulphurous acid and oxygen as it proceeds 

 onwards, until at last it contains no sulphurous 

 acid, and consists almost entirely of nitrogen from 

 the atmospheric air and of peroxide of nitrogen. 

 As will be presently seen, the gas leaving the 

 chambers should contain no nitric oxide, and to 

 insure this, there should be a little oxygen. It is 

 obvious that if the peroxide of nitrogen can be 

 extracted from the effluent gas, it may be used 

 over again to effect the oxidation of a new 

 quantity of sulphurous acid. This extraction is 

 effected in the following way : Peroxide of nitrogen 

 is soluble in strong, but not in dilute sulphuric 

 acid. The effluent gas is, therefore, sent up a 

 tower, or wide chimney, which is packed with frag- 

 ments of coke. Down this tower a stream of strong 

 sulphuric acid is poured, trickling through the 

 coke. The gas is thus freed from the peroxide 



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