50 DECOMPOSITION OF SULPHATE! BY HEAT. 



Two lortiotij fo' ce, aud then they must approach nearer the second. We 

 of acid in me- can conceive two portions of acid in metallic sulphates: one, 

 tjlhc sulphates. j^,j^j^,|^ jg fggbiy retained, tscapes without undergoing de- 

 composition; the other, which is retained more strongly, 

 supports a more elevated temperature, and is decomposed 

 into sulphurous acid and oxigen gas. These two portions of 

 acid, which we may conceive to exist in sulphates, are not 

 the same in all: and it appears, all other circumstances being 

 equal, that, the more soluble a salt is, and the greater its 

 excess of acid, the more sulphuric acid is obtained in its 

 distillation. Hence it is, that sulphuric acid m»y be pre- 

 pared, as is done in Germany, by distilling suVphate of iron 

 or of zinc. The insoluble sulphates would not be any way 

 adapted to this purpose." 

 Allcaline and Mr. Gay-Lussac has extended his researches to the al- 

 *>arthy sul- kaline and earthy sulphates: and he has found, that the 

 ^ ^' salts with excess of acid comport themselves altogether as 



the metallic sulphates; that is to say, they give out sul- 

 phuric acid, sulphurous acid, and oxigen. Those that do 

 not admit an excess of acid do not yield, even if sulphuric 

 acid be added to them, either sulphurous acid or oxigen 

 gas; and nothing is obtained in distilling them but the acid, 

 that exceeded what was requisite for their neutralization. 

 Such are the sulphates of barytes, lime> &c. 

 Roasting of The author, applying all these facts to the roasting of 

 sulphurets. sulphates, concludes. ..,." that> when the roasting is per- 

 fcrmed at a temperature equal to that at which the sul- 

 phates are decomposed, and still more when at a higher 

 temperature, no sulphuric acid will be obtained, and all the 

 sulphur will be given out in the state of sulphurous acid.'* 

 Another mode '* Beside this mode of decomposing sulphates by heat, 

 Tne Tulpha^es ^^'^''^ ^^ another, which is more convenient, as it does not 

 by heat : require so high a temperature. It is that employed by Mr. 



the addition of Gueniveau to decompose the sulphate of lead, distilling it 

 sulphur.t. ^jjjj ^l^g sulphuret of the same metal*. I satisfied myself, 

 that by treating the sulphates of iron and copper in like 

 manner with the sulphurets of the respective metals sul- 

 phurous acid Only was obtained: which proves, 1st, that in 

 this way we may separate the sulphur from metallic sul- 



* See Journal, vol. XVHI, p. 203, 204. 



phurets 



