168 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



He also showed that sulphur could be prepared by heat- 

 ing the solution in a sealed tube during a period of several 

 months (ibid. 124-129). 



Composition of sulphurous anhydride. It was observed 

 by Priestley in 1772 (Experiments on Air, I. 46) and con- 

 firmed by Dalton (New System, II. 391-392) that when 

 sulphur is burnt in air over mercury " no material change of 

 bulk is effected in the gas by the combustion ; and this is 

 also remarked in the analogous combustion of charcoal." 

 The pungent gas, which is now described as SULPHUROUS 

 ANHYDRIDE, thus resembles carbonic anhydride, in that it 

 contains its own volume of oxygen. 



From this observation Dalton was able to calculate 

 the composition of the gas by weight. He had found 

 its density relatively to air to be 2*3, that of oxygen 

 being i'i. The gas was therefore twice as heavy as oxygen 

 and contained approximately equal weights of the two 

 constituents. 



The fact that sulphur gained in weight when burnt was 

 announced to the French Academy by Lavoisier in 1772 

 (Works, II. 103) as the first of his great discoveries in 

 reference to combustion. 



Lavoisier's experiments on the composition of sulphuric 

 acid. Lavoisier recognised that sulphuric acid must be 

 regarded as a compound of sulphurous anhydride with 

 oxygen and with water. He attempted in 1777 (Works, 

 II. 194) to determine the proportions of the two gases by 

 acting on the acid with mercury, and collecting the gas 

 liberated (i) during this action and (2) during the decom- 

 position of the sulphate of mercury into sulphurous an- 

 hydride, oxygen, and mercury. The experiment was not 

 successful because a part of the sulphate of mercury 

 sublimed without decomposition, but it gave clear qualitative 

 proof that " volatile sulphurous acid is a vitriolic acid par- 

 tially deprived of oxygen." 



