66 A'eiv Method of preparivg Alum 



pass through the pipes and collect in the chest called the 

 co>ide?iser, where tliey blend themselves with the lixiviated 

 pyrites. The latter still contain a quantity of highly ox- 

 idated iron, which combines with the sulphuric acid, and 

 forms vitriol. When these pyrites have remained a suin- 

 cient lime in the chest, they are again lixiviated, and more 

 vitriol is obtained from them *. Perfect sulphuric acid 

 would not, in this case, be of the same service as vapours, 

 which are not entirely saturated with oxygen. 



This fact revived my ideas relative to the employment 

 of the vapours which are disengaged during the burning of 

 pyrites; but, before I proceed to state the experiment I 

 made on the fabricaiion of alum, 1 shall introduce an ob- 

 servation concerning the process employed by M. Chaptal. 

 That chemist burns the sulphur with the saltpetre, as is the 

 practice in the English manufactories of sulphuric acid ; he 

 receives the vapours of sulphuric acid, which are disengaged, 

 upon baked clay, and thus forms an artificial ore of alum. 

 Would it not be less expensive to oxidate immediately the 

 sulphur of the pvrites by the atmospheric air? By the latter 

 method two expensive o])erntions would be spared — that o£ 

 purifying the sulphur, and that of burning it with saltpetre; 

 a substance which is at a considerable price. I admit that, 

 in the process which I am going to describe, all the vapours 

 are not turned to advantage ; but if the apparatus be pro- 

 perly disposed, very little will be lost. 



I shall first make a remark concerning the state of the 

 pyrites, and of the clav employed. Pyrites merely broken 

 arc preferable to those which are triturated and washed, when 

 they are to be piled one upon the other: if pyrites in powder 

 be used in the furnaces, they s^liould be mixed with a fourth 

 part of clay, and hardened and dried in the form of bricks-. 

 'I'lie arsenic contained in the pyrites is not detrimental to 

 the formation of the alum, because the arsenic, being less 

 volatile than the vapours of the sulphuric acid, is arrested 

 at the comniencen)ent of the pipes, v.here care must be 

 taken not to ])ut any clay. As to the clay, that used by 

 •potters may be employed, provided it docs not contain too 

 great a quantity of iron. 



I directed a reverberatorv furnace, four feet long, two 

 and a half wide, and two and three quarters high, to be 

 constructed : the anterior part had an aperture a foot square^, 

 by which the pyrites were introduced. On the sides of the 



* Wliat M. L.iir.padius here says h not perfectly correct. Water is made 

 to fall contiiuialiv, drop by drop, into the chest : this water passes through 

 the heap of pyrites, and keeps it conitantiy lixiviated. 



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