44 ON THE PREPARATION OF SODA AND CHLORINE. 



of common crystallized green vitriol, (obtained either by the 

 ordinary methods, or in the manner yet to be described,) are 

 heated until they part with their water of crystallization, and 

 become more or less oxidized, without however losing any sulphuric 

 acid. The calcined green vitriol thus obtained is then mixed with 

 352 parts of common salt, (previously heated until it no longer 

 decrepitates,) and also with *78 parts of peroxide of iron, (which 

 may readily be obtained by calcining pure iron pyrites with five per 

 cent, of common salt) and lixiviating the result. These ingredients, 

 in fine powder, being intimately mixed, are introduced into a 

 muffle calcining furnace, nearly of the same construction as that 

 used in Europe for calcining arsenical pyrites. The mixture is 

 spread over the hearth of the furnace, which is heated to faint 

 redness by a fire placed underneath it. That part of the furnace 

 over the hearth, which may be called the muffle, and to which the 

 flames, or products of the combustion of the fuel have no access, 

 is connected with an exhausting machine, which draws a current 

 of air, (previously dri id, by passing through quicklime), over the 

 mixture which is spread out on the hearth of the furnace. A de- 

 composition takes place at a very low temperature, between the 

 sulphate of iron and the chloride of sodium, sulphate of soda re- 

 sulting on the one hand, and protochloride and perchloride of iron 

 on the other. The temperature in the furnace is so low that nei- 

 ther of the chlorides of iron is sublimed ; but on the other hand the 

 oxygen contained in the dry air passing over the mixture, converts 

 both into peroxide of iron, which remains behind, and chlorine 

 gas, which is drawn off by the exhausting machine. It is of 

 the utmost importance in this operation, that the temperature 

 be kept as low as possible, because anhydrous sulphate of iron 

 and chloride of sodium heated together, to a higher temper- 

 ature, fuse and emit fumes of perchloride of iron. In order 

 therefore to obtain pure chlorine, the mixture must not be per- 

 mitted to fuse, or even to sinter. The peroxide of iron in the 

 mixture has some influence in preventing fusion, and in elimin- 

 ating the chlorine. The greater the quantity of peroxide of iron 

 used, the more easy it is to calcine the mixture without its caking 

 together. I have not found it necessary however to use more 

 than the proportion above stated, when the calcination is performed 

 with proper care. During the operation, the materials are gently 

 but frequently stirred with an iron rake ; this also prevents sin- 

 tering. I have found that in this operation thus conducted. 



