DECOMPOSITION OF SULPHATES BY HEAT. 



45 



process in the small way, and succeeded completely. Re- 

 peating it on sulphuvet of iron, and on a mixture of sulphur 

 and black oxide of manganese, it still afforded me sulphates. 

 The temperature at which these sulphates were roasted was 

 a red heat scarcely visible." 



" The formation of sulphuric acid in the roast- Sulphuric acy 



ing of metallic sulphurets is not peculiar to them. It takes fonned in 

 ptace also, and much more decidedly, in the roasting of a!- line as well as 

 kaline sulphurets. I made some sulphuret of potash, which metallic sul- 

 remained fluid at a low red heat as long as it was kept from i'^"^'^^* 

 the contact of air : but as soon as air was admitted freely, it 

 began to thicken ; and soon after it became solid, because a 

 great deal of sulphate was already formed. I removed it 

 from the fire to powder it, and exposed it anew to the action 

 of heat. In less than an hour it had lost its sulphurous taste, 

 and threw down only a white precipitate with acetate of lead. 

 The sulphuric and muriatic acids extricated nothing from 

 it. The sulphuret of barytes, treated in the same manner, 

 likewise afforded me sulphate; but after three hours roast- 

 ing in a red heat it was still sulphuretted. I examined these They pass ^- 



two alkaline sulphurets, and several metallic sulphurets, at ''^^''y to the 

 . , p . . , , . , , . state of sul- 



vanous periods ot roasting, without ever being able to extri- phates. 



cate sulphurous acid from them. Qonsequently they must 



pass directly to the state of sulphates. 



" It is easy to understand why the alkaline sulphurets Reason of this, 

 pass immediately to the state of sulphates in roasting; for 

 Mr. Berthollft has shown, in the Memoirs of the Academy, 

 that the sulphite of potash is converted into sulphate at a 

 red heat, exhibiting an excess of sulphur and of alkali. On 

 treating sulphite of lead in the same way I obtained a great Oxide of lead 

 deal of sulphurous acid; which proves, that the oxide of ^^"^^ '"o'^® f*^*- 

 lead has a much weaker action than potash on sulphuric acid, a^h on sulphu-' 

 It is probable however, that some sulphate is formed with "c acid, 

 this oxide also ; and if I cannot absolutely affirm this, it is 

 because what I found in the residuum might proceed from 

 sulphuric acid contained in my sulphurous acid. 



" All the metallicsulphurets however arenotequallyadapt- AH metallic 

 ed to livoduce sulphates by roasting. A necessary condition for aiS"r?Uir°* 

 the formation of sulphuric acid is its having a base to com- formation of 

 bine with capable of condensing it sufficiently. I have taken sulphuric acid,, 



sulphuret 



