DECOMPOSITION OF SULPHATES BY HEAT, 49 



the proportioii of 51*5 to 48'5. Though oxide of mercury 

 requires a higher temperature for its reduction than the ox- 

 ide of silver, the sulphate of silver is not so readily decom- 

 posed as that ot mercury. This ditterence may arise in part, 

 no doubt, from the difference in the affinities of the metals 

 for sulphuric acid; but it must depi^nd likewise on the great 

 volatility of the mercury. In general it appears to me, that 

 the affinity, the more or less easy reducibleness of the me- 

 tals, and their volatility, must be considered as so many 

 causes, capable of modifying the action of caloric on their 

 sulphates." 



** From my first experiment with sulphate of lead, not Sulphate of 

 having employed a temperature sufficiently high, I con- 

 cluded, that it was not decomposable by heat. But on 

 having recourse to a reverberatory furnace surmounted with 

 a chimney, 1 obtained its decomposition, and collected a 

 great deal of oxigen gas and sulphurous acid. I did not 

 perceive any lead reduced, or any very sensible quantity of 

 Sulphuric acid. It is very possible, that the separation of 

 the acid was determined by the action of the stone retort, 

 for it was coated internally with vitreous glaze*. Be this as 

 it may, it is evident, that the sulphate of lead, which is inso- 

 luble and without excess of acid, and the decomposition of 

 which cannot be promoted either by the easy reduction of 

 the oxide, or by the volatility of the metal, is much more 

 difficultly decomposed by fire than the acid and soluble 

 sulphates. We may conclude therefore, that the insoluble 

 sulphates resist the action of caloric more than those that are 

 soluble, and that they give out much less sulphuric acid. But 

 to render this conclusion still more general, we must take into 

 consideration the more or less easy reduction of the metals, 

 and their volatility." 



" It may have been observed, that the soluble sulphates Differenca b«, 

 yielded more sulphuric acid, than those that are insoluble. *^^«" *h^ 

 When the former have lost a part of their acid, their salu- 

 blity is diminished, the acid remaining is held with more 



• This suspicioo of the author appears to me well foiinded. The 

 great affinity of the oxide of lead for earths, and particularly for silex, 

 must facilitate the decomposition of the sulphate, if it b« Dot its 

 sole cause. 



Vol. XXXIIL—Sept. 1S12, E force 



