^g PECOMPOSITION OF SULPHATES BY HEAT* 



Suli>huret of sulphuret of tin, a metal that does not combine with $ulphu« 

 *'"» ric acid but very difficultly ; and 1 have roasted it in a red 



heat for an hour, without any thing but sulphurous acid 

 antimony, being produced. In hke manner the sulphurets of anti- 

 bismuth, mony and bismuth, alter having been roasted, presented me 

 only with traces of sutphuiic acid. It niay be remembered 

 too, that, if sulphates of these d-ffeient uietals be distill d, 

 almost all the sulphuric acid pas>es over, as if it had been 

 alone. The affinity of the metal for oxigen also has some 

 aodsilver. influence. When sulphuret of silver is distilled in a stone 

 retort with a strong fire, it is not decomposed ; but if it be 

 roasted, it decomposes with the greatest facility, sulphurous 

 acid only is evolved, and the acid is not oxided. 

 Importsnt clr- " Thus, then, an important circumstance, the condensa- 

 cunistance, ^-^^^ ^f ^1^^ acid, modifies the phenomena presented by the 

 the conaensa- . ^ • 

 tionofthe metallic sulphurets in roasting. When the noetals have the 



*'='^- property of combining with sulphuric acid, and causing it to 



undergo a certain degree of condensation, sulphates are al- 

 ways formed. When, on the contrary, thev combine but 

 very difficultly with it, sulphurous acid only is foinied, 

 which flies off, as its great elasticity cannot be overcome by 

 the affinity of the metalhc oxides." 



We will now proceed to the decomposition of the sul- 

 phates, which is the principal subject of the paper. 



Common "It was supposed," says Mr. Gay-Lussac, " that, 



theory of the ^^ distilling a metallic sulphate, sulphuric acid was obtained,. 



distillation of . " r ' r 



sulphates if the oxide were not susceptible of a farther degree of oxi- 



dation ; or sulphurous acid, if its oxidation might be carried 

 farther. It was thought too, that all the alkaline and 

 earthy sulphates with excess of acid were brought back to 

 the neutral state by the action of caloric, or entirely 

 decomposed, yielding as the result only sulphuric acid. 



erroneous. This theory is not the expression of facts accurately 

 observed." 



Effects of " The first sulphate subjected to the action of heat 



heat on sul- ^jjg ^j^g^^ ^f copper. Water first passed over : but as soon as 



copper. the retort began to grow red, white vapours of sulphuric 



acid arose, accompanied with a nebulous gas; smelling 

 strongly of sulphurous acid, and in which a match kindled 

 several times following, when it had been washed. This gas 



therefore 



